

FORM OF THE EMBRYO AT DIFFERENT STAGES OF ITS GROWTH 75 



pletely closed amnion. It possessed a minute yolk-sac and was surrounded by mesoderm, which 

 was connected by a band to that lining the trophoblast (Fig. 32). * 



Second Week. By the end of this week the ovum has increased considerably in size, and the 

 majority of its villi are vascularized. The embryo has assumed a definite form, and its cephalic 

 and caudal extremities are easily distinguished. The neural folds are partly united. The embryo 



Mid-brain 



Fore-brain 

 Stomodeum 

 Mandibular arch 

 Heart 



Hind-brain 



Auditory vesicle 



Visceral 

 arches 



Amnion (cut} 



Body-stalk 

 Fio. 59. Human embryo between eighteen and twenty-one days old. (His. 



more completely separated from the yolk-sac, and the paraxial mesoderm is being divided into 

 the primitive segments (Fig. 58). 



Third Week. By the end of the third week the embryo is strongly curved, and the primitive 

 segments number about thirty. The primary divisions of the brain are visible, and the optic 



Heart 



Fore-limb 



- 





Hyoid arch 



Mandibular arch 

 Maxillary process 



Eye 



Olfactory pit 



Chorion ' 



<%m&L^ t 



Hind-limb 

 FIG. 60. Human embryo, twenty-seven to thirty days old. (His.) 



and auditory vesicles are formed. Four branchial grooves are present: the stomodeum is well- 

 marked, and the bucco-pharyngeal membrane has disappeared. The rudiments of the limbs 

 seen as short buds, and the Wolffian bodies are visible (Fig. 59). 



1 Bryce and Teacher (Early Development and Imbedding of the Human Ovum, 1908) have described an ovum which 

 they regard as thirteen to fourteen days old. In it the two vesicles, the amnion and yolk-sac, were present, but there 

 was no trace of a layer of embryonic ectoderm. They are of opinion that the age of Peters' ovum has been understated, 

 and estimate it as between thirteen and one-half and fourteen and one-half days. 



