353 



disc which represents the tail of the embryro is now so far removed 

 that the anterior portion reaches a high degree of differentiation before 

 this part of the blastodisc can pass over the yolk and coalesce, or 

 concresce, to form the remainder of the body. Indeed this movement 

 over the yolk has been so much retarded that the farthest removed 

 portion at 1. becomes cut off, as shown in the diagram, and never 

 enters into the formation of the embryo. The part left behind gives 

 riso to the yolk blastopore which Balfour found in the Elasmobranchs. 

 Miss. Clapp observed the same process in Batrachus as did also 

 Kauber and Whitman in the Chick. The relative retardation in the 

 closure of the blastopore in these forms helps us to understaud the 

 appearance in the blastodermic margin of neuromeres in that the lateral 

 portions of the germ-ring are unable to unite and form the embryo 

 at the proper time and as a result the differentiation of structures 

 proceeds along the separated halves of the embryo. These observations 

 moreover show that the caudal end of the embryos of the Amphibia 

 and Teleosts are not homologous with the caudal end of the Elasmo- 

 branchs and Birds. 



We may then summarize by stating that in those Amphibia which 

 approach most nearly the holoblastic type, as Rana, Bufo, Acris and 

 Chorophilus, the greater portion of the embryo is formed through 

 differentiation in situ and overgrowth, concrescence being confined to 

 a limited region at the caudal end of the embryo. In those forms 

 like Necturus in which there is a marked raeroblastic tendency, due 

 to the relative increase in the amount of yolk, a lesser extent of the 

 embryo is formed through differentiation in situ while there is a 

 corresponding increase in the extent of the embryo formed through 

 concrescence, or coalescence of the lateral margins of the blastopore. 

 Passing on to the condition found in most Teleosts in which the 

 meroblastic character is more pronounced, through a still greater 

 relative increase in the amount of yolk, we find a further decrease in 

 the extent of the embryo which is formed in situ, with an increasing 

 extent formed through concrescence. When we finally consider those 

 forms in wich there is a maximal amount of yolk, as in the Elasmo- 

 branchs and Birds, we observed that difierentiation in situ forms a 

 limited region only at the cephalic end of the embryo; while concres- 

 cence gives rise to by far the greater portion of the embryo. 



In view of the above facts there is every reason for maintaining 

 that difierentiation in situ is the primitive method of embryo form- 

 ation, concrescence being a secondary process which has progressed 

 pari passu with the increase of yolk material. 



Anat. Anz. XXI. Aufsätze. OQ 



