132 



EMBRYOLOGY. 



inner germ-layer (ik). The place where the bend occurs even pro- 

 trudes distinctly as a lip into the ccelenteron, as in the case of the 

 Amphibia. Except for these unions at the sides of the chordal 



mk 1 mk" ch 



Fig. 95. Cross section through the germ-disc of an embryo Rabbit, after E. VAN BENEDEN. 

 "k, Outer, ik, inner, mk, middle germ-layer ; ink 1 , parietal, mk~, visceral lamella of the middle 

 genii-layer; ch, chorda. 



fundament, the middle germ-layer is everywhere sharply separated 

 by a fissure from the other two germ-layers.* 



Further agreement with the conditions which the investigation 

 of Triton has furnished is afforded by a series of cross sections 

 through the primitive streak the obliterated blastopore. In the case of 

 all Vertebrates, this is the only place in the whole embryonic area where 

 all three germ-layers, although for only a short distance, are fused with 

 one another, and cannot be distinguished as separate layers, whereas at 

 the sides of this region they are separated by distinct fissures. 



gr 



pr 



mk 



a/c 



Fig. 96. Cross section through the middle of the primitive streak of a Chick's germ-disc, which 

 is in the stage of development represented in Fig. 81 B. After ROLLER. 



At some distance from the primitive groove is to be seen upon the left side of the figure in cross 

 section the marginal groove of His. Upon the right side it is as yet little developed. 



ale, Outer, ik, inner, mk, middle germ-layer ; pr, primitive groove ps, primitive streak ; 

 gr, marginal groove. 



Figure 96 represents a cross section through the embryonic area 

 of a Chick in which the primitive groove is distinctly developed, 



* In the development of Mammals there has been observed at certain stages 

 under the fundament of the chorda a peculiar structure, the so-called chordal 

 canal, which is not found in the other classes of Vertebrates. I mention it 

 here only incidentally, because the publication of VAN BENEDEN'S investiga- 

 tions will doubtless furnish the desired explanation of its origin and signi- 

 ficance. 



