84 



GENERAL HISTORY OF DEVELOPMENT 





mesodermic segments are distinct, and ventral to them on each side is a longitud 

 ridge called the Wolffian ridge. This ridge is more prominent at two points, 

 opposite the posterior end of the heart and allantoic stalk ; the small swellings 

 thus produced are the earliest signs of the limb-buds. 



The general features of a f cetation at the end of the first month are well illustrated 

 in fig. 116. The chorionic vesicle is laid open. It is still very large relatively 

 to the contained embryo. The yolk-sac is a rounded vesicle attached to the 

 embryo by a stout pedicle, and, owing to the flexion of the embryo, the abdominal 

 stalk and vitelline stalk come to be applied to one another. The embryo is closely 

 invested by the amnion, and is markedly bent on itself. This bending is best 

 seen about the twenty-third day, when head and tail touch or even overlap one 

 another. The greatest diameter of the embryo by the end of the fourth week is 

 about 7*5 mm., so that, allowing for the flexion, it has obviously much increased 

 in size. The cervical flexure is very marked. 



Fig. 119 represents an enlarged view of an embryo at this stage. On the side 

 of the head are seen the olfactory pits, above them are the optic vesicles ; the lens- 

 rudiments have the form of shallow open pits. The auditory vesicles are prominent 

 rounded swellings, opposite the hyoid arches. The mouth is now a wide cavity 



bounded in front (fig. 123} 

 by a broad field inter- 

 vening between the olfac- 

 tory pits called the fronto- 

 nasal process, behind by 

 the mandibular arches, and 

 on each side by lateral 

 processes named the 

 maxillary processes, which 

 project forwards between 

 the optic vesicles and 

 mandibular arches. The 

 mandibular and hyoid 

 arches are each beginning 

 to show swellings separ- 

 ated by constrictions, and 

 view, being overlapped by the 

 a recess between the 



heart 



f> 



yolk- stalk 



dbdom. stalk 



FIG. 



115. EMBRYO OF 42 MM. (LETTERED Lr) EIGHTEEN 



TO TWENTY-ONE DAYS OLD. (His.) 



and. ves., auditory vesicle ; be, stomodoeum ; fb, fore-brain; 

 mb, mid-brain ; lib, hind-brain. 



the hinder branchial arches are hidden from 



anterior arches. This telescoping of the arches produces 



branchial region and trunk, known as the precervical sinus. 



The mesodermic segments are very prominent objects ; they are thirty-five in 

 number. In the early part of the fourth week the paired swellings on the Wolffian 

 ridges become more prominent, and by the end of the week are seen as distinct 

 buds, the rudiments of the limbs. The heart is relatively very large, and, with the 

 developing liver behind and slightly above it, forms the prominent rounded swelling 

 represented in the figure. 



It will thus be seen that by the end of the first month all the organs have been 

 laid down, and the embryo closely resembles any other mammalian embryo at a 

 corresponding stage as, for instance, the rabbit embryo at the end of the eleventh 

 day. During the course of the second month, however, changes take place, which 

 confer distinctively human features on the embryo. 



Second month. During the first month the chorionic vesicle is, as we have 

 seen, relatively very large compared with the embryo and its amnion, and the 

 villi are uniformly distributed. By the end of the second month the distinction 

 between chorion frondosum and chorion Ia3ve is established, and the amnion has 

 become greatly enlarged, so as to come in contact with the chorion and obliterate 



