U2 EMBRYOLOGY. 



fig. 70 by stars ( * *), grow toward each other under the chorda and 

 fuse into a median raphe. 



The final result of all these processes is shown in the cross section 

 fig. 72 : the original coelenteron has become divided into three cavities 

 into the ventral permanent intestine (dh), and into the two body- 

 cavities (/A), which are situated dorso-laterally to it, and which con- 

 tinue to increase in size. Between these there has been interpolated 

 the chorda (ch), upon which the intestine abuts below and the neural 

 tube (n) above. The cells which have been cut off from the coelen- 

 teron by constriction and which are more deeply shaded in figs. 69 

 to 72, and enclose the body-cavities (Ih) constitute the middle 

 germ- layer (mk). The part which lies in contact with the outer 

 germ-layer (fig. 72) is recognisable as the parietal middle layer 

 (mk 1 ) ; the part which is in contact with the neural tube, chorda, 

 and intestine as the visceral middle layer (mk 2 ). 



Inasmuch as the process of differentiation just described begins^ 

 as has been already stated, at the front end of the embryo and 

 extends slowly step by step toward the hind end, by an examina- 

 tion of a series of sections one may follow the various stages of 

 metamorphosis on a single object. 



In the description given I have presented the conditions as though 

 in Amphioxus there arose two simple body-sacs, one on either side 

 of the intestinal tube. The processes are, however, somewhat more- 

 complicated, for in the case of the embryo of fig. 70 the body-sacs, 

 while increasing in size posteriorly, undergo further changes in the 

 .interior region, and through repeated infoldings are divided into- 

 separate compartments, the primitive segments (us), which lie one 

 behind the other. I content myself with this statement, since for 

 didactic reasons I shall defer the treatment of the development of 

 the primitive segments until I come to a subsequent chapter. 



While in the case of Amphioxus lanceolatus there is no doubt but 

 that the body-cavity and the middle germ-layer are formed by an out- 

 pocketing of the wall of the cwlenteron, opinions upon the origin of the 

 same parts in the case of the remaining Vertebrata are still very 

 divergent. This results, in the first place, from the fact that the in- 

 vestigation, which can be carried out only by means of serial sections, 

 is coupled with greater technical difficulties, and, secondly, because tne 

 conditions are somewhat altered, owing to the greater abundance of 

 yolk in the eggs, and furnish less clear and intelligible views. Wheiv 

 in the pastrula of Amphioxus a great cavity is present, we see in the 

 case of the remaining Vertebrates a great mass of yolk-material 



