GASTRULATION AND CONCRESCENCE. 35 



derm. This stage represents the radiate or ccelenterate phase, and is to be re- 

 garded as the true gastrula of the arthropod-vertebrate stock. The central de- 

 pression deepens and later forms the stomodgeum, the outer opening being the 

 neurostoma. The stomodaeum, at a very early period, is enclosed, or surrounded 

 by a nerve ring consisting of the stomodaeal ganglia and their commissures, which 

 probably represents the remnants of the circumoral nerve ring of the ccelenterates. 

 The stomodasum is, therefore, caught in a trap, the bars of which are continually 

 growing stronger, and from which it never escapes. 



Transition from Radiate to Bilateral Symmetry. The blastodisc, or 

 cell layers covering the primitive cumulus, gradually spreads out over the surface 

 of the yolk in all directions. Then, on the posterior side of the disc and independ- 

 ently of the central depression, a second thickening appears, in which cell growth 

 and proliferation is especially active. It marks the beginning of apical growth 

 and of bilateral symmetry, and lays the foundations for the first metameres. The 

 anterior portion of the primitive cumulus gives rise to the procephalic lobes. 

 (Figs. 24, 25.) 



The Telopore. The rapid cell division at the apex of the developing trunk 

 may give rise either to an elongated axial groove (insects), or to a terminal in- 

 folding, or telopore (arachnids), the so-called ''blastopore" of authors. Later 

 it may be changed to a typical primitive streak (Limulus and scorpion), or there 

 may be no infolding whatever (Cymothoa). This axial or terminal ingrowth 

 (Fig. 25), is not to be regarded as a modification, or as an extension of the process 

 of gastrulation in the procephalic lobes. It is merely a local exaggeration of the 

 marginal growth of the blastodisc. The infolding is a secondary result of the 

 rapid tangential proliferation that takes place at the head of the comet-like out- 

 growth. It may or may not be present. 



The Germ Wall. At the close of the primitive cumulus stage, a germ wall, 

 g. w., is formed on the lateral and posterior margins of the blastodisc. With the 

 formation of the trunk, it forms the lateral boundaries of the developing metameres. 

 (Figs. 25, 26). The germ wall, which is merely a thick band of proliferating 

 cells, similar to the teloblasts at the caudal end, spreads laterally over the yolk 

 surface (Figs. 31, 32), leaving behind, in addition to the ectoderm and yolk 

 cells, a sheet of underlying mesoderm that gradually breaks up into somites and 

 lateral plates. 



Differentiation, therefore, takes place along two main axes; from the median 

 line laterally, and from the head end backward; hence the anterior median part 

 of the germinal area, which now includes the primitive head and the new body, 

 is always the oldest and shows the greatest histological differentiation; the marginal 

 and caudal part is the youngest and is the least differentiated. 



Concrescence of the Germ Wall. As the gradually widening germinal 

 area advances over the yolk, the germ walls of the more posterior segments form 

 a A , the arms of which gradually unite, forming a double, primitive streak-like 

 band of nuclei behind the telopore. (Figs. 16, 21, 25, 26, 27, 138.) 



