Timeline of embryonic development: Weeks 7 and 8

00:01 Entering Week 7, day 43, crown-rump length is around 11.0 to 14.0 mm, and now, in addition to muscle masses, we're getting limb cartilages forming in the mesenchyme of the upper and lower limb.

00:15 By day 44, crown-rump length is extended at 13.0 to 17.0 mm and facial development continues.

00:22 The upper lip continues to get smoother and smoother, the nasal prominences are fusing with the maxillary prominences and the eyes are beginning to migrate.

00:31 The eyelids are forming at the same time and will eventually close and then reopen.

00:37 By day 45, crown-rump length has reached 13.0 to 17.0 mm and the muscular ventricular septum has met the membranous ventricular septum growing down from the endocardial cushions and we have a completely separate left and right ventricle.

00:54 The head, has continued to enlarge and is quite huge in comparison with the rest of the embryo but the chin is a little bit recessed and will continue to expand as facial development proceeds.

01:06 The digital rays are visible in the hand plate and will only become more distinct as the webbed skin between adjacent digital rays undergoes apoptosis, dies off, and makes the fingers more and more visible.

01:20 By day 46, crown-rump length has reached 16.0 to 18.0 mm and the embryo and its associated membranes have grown so large that it's bulging out into the lumen of the uterus creating distinct decidual layers such as the decidua basalis and parietalis.

01:37 Inside the heart, the atrial septum is formed and now the foramen ovale and the foramen secundum are positioned in such a way that the valve of the foramen ovale is allowing blood from the inferior vena cava carrying oxygen to move through the right atrium cleanly to the left atrium from where it will be pumped out to the rest of the body.

01:57 By day 47, crown-rump length is roughly 16.0 to 18.0 mm.

02:02 The limbs begin to rotate with the elbow rotating posteriorly and the knees rotating anteriorly.

02:10 The decidual layers into the lumen of the uterus are very clear and are only gonna become more pronounced as the embryo takes up more and more space within the uterus.

02:20 The external genetalia are visible and present but cannot yet be distinguished as male or female.

02:27 We can see the genital tubercle, the urogenital membrane, and anal membranes present but at the moment, they have not yet diverged into distinctly masculine or feminine appearance.

02:38 By day 48, crown-rump length is between 16.0 and 18.0 mm.

02:44 The medial, lateral, and maxillary prominences of the face have completely fused creating a more or less, completely human appearance to the face.

02:53 The midgut has herniated into the umbilical cord completely and is gonna hang out there until we've entered the fetal period at which point the abdominal cavity will expand at a point that the midgut can move back in.

03:06 The urogenital sinus and rectum have been completely separated by the urorectal septum that has grown down to form the perineal body on the surface of the embryo's perineum.

03:17 By day 49, crown-rump length has reached 18.0 to 22.0 mm.

03:22 The eyelids have completely formed but have not yet met each other and cover the eyes.

03:27 The external ear has formed and looks more or less like an ear.

03:31 It has not yet reached the mature position on the head but it is visible.

03:35 The secondary palate has begun to fuse inside the mouth and extend posteriorly towards the nasal pharynx and digits of the hands and feet are now distinct and the webbing between them is becoming very, very thin.

03:49 Entering week 8, day 50, crown-rump length has reached 18.0 to 22.0 mm.

03:56 The upper limbs are completely flexed at the elbow and the lower limbs are flexed at the knee.

04:02 Day 51, we've got the length between 22.0 and 24.0 mm, and the facial features are now completely distinct.

04:10 Day 52, we're between 22.0 and 24 mm in length.

04:15 Fingers and toes are now distinct and the webbing has more or less completely disappeared from between them.

04:21 Day 53, crown-rump length 23.0 to 28.0 mm and the external genitalia have begun to appear as male or female.

04:30 It may not be easy to tell the difference by ultrasound yet, but it is theoretically possible to determine the sex of the embryo at this point just from ultrasound examination.

04:41 By day 54, we're now between 23.0 and 28.0 mm in length, the genital tubercle, urethral groove, and anus are completely visible to ultrasound and as mentioned before are gonna become more distinct as time progresses.

04:55 Day 55, between 23.0 and 28.0 mm in length, the conotruncal ridges in the outflow tracts of the heart have begun to form and separate.

05:06 Those regions of the developing heart into a distinct aorta and pulmonary trunk.

05:11 And by day 56, at 27.0 to 31.0 mm of length, the inner, middle, and external ear structures are all present.

05:21 The limbs have definitely reached their final orientation and the embryo, basically looks like a very tiny person.

05:29 And we have reached the end of the embryonic period and the beginning of the fetal period.

05:34 Thank you very much and we'll see you for the very next and final talk.