120 



EMBRYOLOGY. 



Fig. 80 Longitudinal [sagittal] section through an advanced em- 

 bryo of Bombinator, after GOETTE. 



m, Mouth ; an, anus ; I, liver ; ne, neurenteric canal ; me, medullary 

 tube ; ch, chorda ; pn, pineal gland. 



other, and thus cause to appear a right and a left body-cavity 

 (enteroceel), which, according to my interpretation, were not pre- 

 viously recognisable, simply on account of the intimate mutual 



contact of their 

 walls. 



Meanwhile the 

 medullary plate 

 has become con- 

 verted, by the 

 process of folding 

 already described, 

 into the neural 

 tube (fig. 80 me), 

 which lies beneath 

 the epidermis. 

 Since the neural 

 tube subsequently 



encloses the blastopore, and is thereby in communication with the 

 intestinal tube (as the preceding longitudinal section of an advanced 

 embryo of Bombinator most distinctly shows), it follows that there is 

 also in the Amphibia a structure (fig. 80 ne) corresponding to the 

 neurenteric canal of Amphioxus (compare fig. 68 en). 



More fundamental differences in the development of the middle 

 germ-layer are 

 met with in 

 the eggs of 

 Fishes, Eep- 

 tiles, and Birds, 

 which are more 

 abundantly 

 provided with 

 nutritive yolk 

 and undergo 

 partial cleav- 

 age, and also 



in the eggs of Mammals. However, the variations appear in these 

 cases to be of a subsidiary nature, whereas in the chief points the 

 unity of the developmental processes for all vertebrated animals has 

 been the more firmly established the more accurately the individual 

 stages have been investigated by means of improved methods. 



In the presentation of these difficult conditions, we shall describe 



^ 



Fig. 81 A and B.-Two germ-discs of Hens' eggs in the first hours of 



incubation, after KOLLF.R. 

 df, Area opaca; hf, area pellucida ; s. crescent; sk, crescent-knob; 



Es, embryonic shield ; pr, primitive groove. 



