220 EARLY STAGES OF ARTHROPOD AND VERTEBRATE EMBRYOS. 



new type of radial structure (echinoderms) , without thereby destroying or com- 

 pletely disguising the basic elements of bilateral structure and of metamerism. 



In all these varying forms, the part of the embryo that represents the coelen- 

 terate ancestor is the head, and the primitive infolding, or ingrowth in the center 

 of the head region, represents the remnants of the coelenterate enter on. This is the 

 only part of the embryo that may be properly called a gastrula. The opening of 

 the gastrula becomes the opening to a subsequent ingrowth, the stomodaeum; or 

 the stomodaeum is formed, as nearly as may be determined, at the point where 

 the gastrula infolding, or the so-called blastopore, closed. 



That is, in bilaterally symmetrical, or in segmented animals, or in their deriva- 

 tives, the true gastrula is formed at the head end only, and the permanent mouth is 

 formed from it, or in its place. When a germinal ingrowth forms in the post- 

 cephalic region, it is not to be regarded as a blastopore, but as a telopore, or as one 

 of the various stages of the axial infoldings that arise as a secondary result of 

 apical growth. 



A coelenterate type of animal represented in mercator projection, or as laid 

 down in enbryo on a large yolk sphere, would take the form of a thin circular disc, 

 with the nerves, sense organs, and appendages arranged in concentric and radiat- 

 ing lines around a central, enteric infolding, or ingrowth. (Fig. 119, A.B.) 



In the arachnid embryo, the coelenterate stage is represented by the circular 

 germinal disc, with its central infolding. (Figs. 123-124.) This disc becomes 

 a part of the head, or procephalic lobes of the future embryo, while the trunk is an 

 outgrowth from its posterior margin. 



In the later stages, we may recognize the remnants of the primitive, radiate 

 nervous system, in the system of radiating and concentric nerves formed from the 

 walls of the stomodaeum. If we evert the stomodaeum, and project it with its 

 nerves on the central area of the germ disc, where it originally belonged, the 

 radiate arrangement of the stomodaeal nerves is apparent. (Fig. 35.) 



In vertebrates, a part of the stomodaeal ring is still recognizable in the cere- 

 bellar commissure and the side walls of the diencephalon, while the location of the 

 blastopore, and of the invertebrate stomodaeum is indicated by the infundibular 

 perforation of the brain floor, and by the pit-like infolding in the center of the 

 procephalic lobes during the open medullary plate stage. (Figs. 25, st, 46, st.co.) 

 During the gastrula stage of the metacoelente rates, therefore, the primitive mouth, 

 or blastopore, is surrounded by a closed nerve ring like that of their coelenterate 

 prototype. On this interpretation, the union of the margins of the germinal disc 

 represents the closing of a yolk navel on the haemal surface, not the concrescence 

 of the blastopore on the neural surface. Such a concrescence of the blastopore is 

 only conceivable on the untenable assumption that the yolk sphere completely 

 fills the gastrula mouth, and that the neural surface of segmented animals repre- 

 sents the aboral or haemal surface of a coelenterate. 



Wherever yolk is present in small amounts in the egg, it may be held within 

 the endodeim cells. But if it is very voluminous it forms an inert, extra-cellular 



