136 



EMBRYOLOGY. 



to be answered : (1) What is tlie meaning of the primitive groove? 

 (2) How is the middle germ-layer developed ? 



, In the interpretation of the primitive groove I place myself, as is 

 to be seen from what precedes, wholly on the side of those investi- 

 gators who, like Balfour, ITATScnEK, Kdpffer, Hoffmann, van 

 Beneden, L. Gerlach, EUckert, and others, recognise in it a structure 



equivalent to, but somewhat modi- 

 fied from., the blastopore of lower 

 Vertebrates, and who compare the 

 primitive Jolds to lateral hlasto- 

 poric lips closely pressed together. 

 In my description of a previous 

 stage I have already designated 

 as blastopore the crescentic 

 groove of Birds (fig. 52 £ s) 

 and the prostoma (fig. 55 u) 

 of Reptiles, because that is the 

 place where the lower gsrm-layer 

 is infolded. In my opinion both 

 grooves are identical structures, 

 which, by changes in position and 

 form, have been so evolved, the 

 one from the other, that the 

 fissure, which was at first trans- 

 verse, has become converted info a 

 longitudinal one. For Reptiles 

 KuPFFER has established this to 

 a certainty. According to his figures in Bmys Europsea, e.g., the 

 transverse depression (u) represented in fig. 101 A is converted at 

 a later stage into the form shown in the adjacent figure (101 £ u). 

 For the Birds the investigations of Duval previously recounted 

 (p. 121, fig. 82) are convincing. There is also to be taken into 

 account the additional fact, that even as early as in the 

 AmphiDia an exactly corresponding metamorphosis of the blasto- 

 pore takes place. As the accompanying cuts (fig. 101 C and D) 

 show, the blastopore of the Amphibian is, at its first appearance, 

 a transverse fissure (fig. 101 C u). Then it becomes circular, and 

 embraces with its lips a protruding portion of the otherwise 

 enclosed yolk-mass, — the yolk-plug, — becomes narrower, and is 

 continued forward into a longitudinal groove. Finally it appears 

 (fig. 101 D u) as a deep groove- .situated at the end of the 



Fig, 100. — Cross sections through the posterior 

 end of a young embryo of Lacerta muralis, 

 nftar Balfour. 



In figure A the nenrenteric canal is cut length- 

 wise ; in figure B only an evagination of 

 it, which is directed backward. Since the 

 sections probably have not cut the chief 

 axis of the embryo perpendicularly, the 

 middle genn-layer is fused with the wall 

 of the canal only on the right side in figure 

 A, whereas in figure B the connection is 

 present on both sides. 



\e, Neurenteric cana ; ep, outer, ^nep, middle, 

 ftT/, lower germ-layer. 



