2 Growth in length of the Vertebrate Embryo 



Oddly enough it is also to embryos of this type that those 

 who oppose this view can best turn for their experimental evidence 

 of a destructive character against the theory. 



The theory of concrescence is as we shall see more difficult to 

 combat with reference to the Amniota, for in them the theorv 



V 



confronts us in a more subtle form. 



The eggs of most of the members of these two classes the 

 Elasmobranchs and Teleosteans are large and are heavily laden 

 with food yolk. 



After the egg of an Elasmobranch has undergone a partial or 

 meroblastic segmentation,, a little disc or cap of cells is formed 

 upon one pole. This we may call the upper pole, because in most 

 cases, though not in all, the egg floats with the pole bearing this 

 cap uppermost. 



The cap of cells soon begins to expand and to become differen- 

 tiated into distinct layers, an outer or upper one which is a 

 continuous membrane, and is called the ectoderm or epiblast; 

 and an inner tissue composed of loose cells and a yolk mass which 

 together may be called the endoderm or hypoblast. This inner 

 layer is continuous with the outer one peripherally, but at the 

 posterior end it is continued inwards for a short distance and then 

 becomes continuous with the yolk mass. The whole disc derived 

 from the segmented region of the egg is called the blastoderm or 

 blastodisc, the rest of the egg is called the yolk mass. This is the 

 gastrula stage, and the narrow shallow cleft, which in the Elasmo- 

 branch extends under the edge of the blastoderm, is really the 

 primitive gut cavity, and it may be called the archenteron. The 

 edge of the blastoderm, where the ectoderm and. endoderm are 

 continuous with each other is therefore the blastopore lip; and 

 the great yolk mass must be regarded as a piece of the floor of the 

 gut projecting out of the gastrula mouth. 



The diagram, Fig. 1, is really a compromise between the 

 Elasmobranch and the Teleostean condition. In the Elasmobranch 

 there is a much greater difference in size between the blastoderm 

 and the yolk mass than in the Teleostean. 



The Teleostean furthermore is complicated by the presence of 

 an outer envelope which is split off from the ectoderm layer of 

 cells. This layer is at first continuous with the yolk, but at a 



