120 



EMBRYOLOGY. 



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

 (enterocoel), 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 



B 



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 ; jm, pineal gland. 



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 jd^erences jn the^ development of the middle 

 germ-layer are 

 met with in 

 the egg's of 

 Fishes, Rep- 

 tiles, and Birds, 

 which are more 

 abundantly 

 provided with 



nutritive yolk F . & gl A ftnd B _ Two germ . discs of H ens' eggs in the first hours of 

 and Undergo incubation, after ROLLER. 



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



E, embryonic shield ; pr, primitive groove. 



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 l.as 

 been the more firmly est a hi ished (lie more accurately the individual 



haw ln'Mi investigated by means of improved methods. 

 In the presentation of these difficult conditions, we shall describe 



df 



.-- hf 



--"-* PT 



