82 



GENERAL HISTORY OF DEVELOPMENT 



neural groove V 



r. 



neurenteric canal -ii c 



P 



I 



primitive streak V 



abdominal stalk 



FIG. 111. EMBRYO OF 2 MM. (LETTERED Gle\ ABOUT THIRTEEN 

 DAYS OLD. (After Graf v. Spee, from Kollmann.) 



the connecting stalk at the posterior end. The fore-gut is beginning to be formed, 



and the allantoic diverticulum is a distinct tubular passage. The heart, as already 



described, is represented by 

 two lateral vessels which unite 

 together in front of the neural 

 groove. 



His' embryos E and S R 

 (fig. 112), estimated to be 

 about thirteen days, and 

 Eternod's embryo present the 

 same characters. 



Kollmann's embryo (fig. 

 113), estimated as fourteen 

 days old, is farther ad- 

 vanced. It measures 2'4 

 mm. in length, and is 

 separated both in front and 

 behind from the yolk-sac. 

 The neural groove is closed 

 behind, but open in front, 

 where the cerebral vesicles 

 form a conspicuous feature. 

 There are fourteen meso- 

 dermic segments, and the 

 heart is now a single coiled 

 tube. The embryonic axis is 

 slightly concave. 

 By the thirteenth day, as illustrated by His' embryo lettered Lg (fig. 114), 



the chorionic vesicle has enlarged to 15 mm. by 12 '5 mm., and the villi are 



numerous and branched. (In 



this particular case they were ^ 



absent from patches at the 



poles.) The amnion closely 



surrounds the embryo, which 



in most specimens shows a 



remarkable dorsiflexion. This 



flexure may possibly be a 



normal 1 though passing fea- 

 ture, as it is seen also in 



some of the lower primate 



embryos of the same stage of 



development. The yolk-sac 



is spherical and about 2 mm. 



in diameter. The fore-gut 



and hind-gut are formed, but 



the cavity of the sac has still a 



wide mouth. The neural groove 



is closed, and the fore-brain is 



bent downward to form the 



cranial flexure ; the optic vesicles project from the sides of the fore-brain ; the eai 



vesicles are seen as distinct pits above the branchial region, which is marked by two 



1 An embryo of 2'5 mm., and estimated as about fifteen days old, studied and modelled by Dr. Petei 

 Thompson, does not show this dorsiflexion (Jour. Anat. and Phys. xli. April 1907). 



amnion 



aMom. 



yolk-sac 



FIG. 112. EMBRYO OF 2'2 MM. (LETTERED SB) TWELVE 



FIFTEEN DAYS OLD. (His.) 





