FORMATION OF GERMINAL LAYERS IN CHELONIA. 41 



continuous with the undifferentiated cells forming the*posterior 

 wall of the blastoporic passage. From the ectoblast in the 

 median line behind the yolk-plug, cells begin to proliferate and 

 constitute the primitive streak. This may happen before the 

 blastoporic passage is completed (see Strahl, No. 8, Taf. xiv, 

 fig. 11). Proliferation begins also from the posterior wall of 

 the blastoporic passage. We shall then have a stage exactly 

 like that given in figs. 1 a and b. When we make a careful 

 study of the latter embryo, some such series of changes as we 

 have sketched out will become an absolute necessity. Our 

 views are in the main like those of Weldon and of Strahl, 

 but we think we have filled in more details. Strahl, it is 

 true, says that the passage begins in the middle of the primi- 

 tive streak. We are inclined to think that in his figs. 8 and 9, 

 Taf. xiv (No. 8), he has stages in which the diflFerentiation of 

 the ectoblast from the entoblast has not proceeded as far as the 

 dorsal lip of the blastopore. In our view, the 2nd and 3rd 

 processes given in his account have the closest relations to each 

 other. Our hypothesis also makes what takes place in Reptilia 

 harmonise well with the development of lower forms, especially 

 of the Amphibia. 



Discussion of the Results of our Observations. 

 In an article published as early as 1875 Balfour (No. 1, 

 p. 208) states that "Amphioxus is the Vertebrate whose mode 

 of development in its earliest stages is the simplest, and the 

 modes of development of other Vertebrates are to be looked 

 upon as modifications of this, due to the presence of food ma- 

 terial in their ova." In the same article, as well as in several 

 subsequent publications (Nos. 2 and 3), he endeavoured to 

 work out the comparison of the vertebrate development with 

 the idea given in the above quotation for its foundation. Above 

 all, he has insisted that the mesoblast always arises as paired 

 masses, one on each side of the median line, and that these 

 two masses are to be regarded as paired diverticula of the ali- 

 mentary canal. Recently O. Hertwig (No. 6), in connection 

 with the "Coelomtheorie" of himself and his brother, has worked 



