404 



THE CRANIATES AND THE ACRANIATES. 



plication of cells that lie beyond the limits of the true blastopore, that is on the 

 posterior margin of what represents the body of the gastrula. The bands of new 

 tissues, or organs, formed by the teloblasts, such as the nerve cords, mesoderm, 

 and entoderm, are usually quite distinct and preserve their normal position and 

 relation to one another from the outset. But as a result of diverse conditions 

 created by growth, the entire mass of proliferating cells may be bodily invaginated 

 (insects, Crustacea, amphibia) forming extensive axial, or terminal, infoldings 

 from which the products of apical growth are gradually separated. (Fig. 269, t.p.) 

 These infoldings, so often confused with gastrulation, are of a purely secondary 



FIG. 268. Diagrams of a molluscan trochosphere ; A ,B, in sagittal section ; C, D, seen from the neuralor oral surface. 

 The diagrams indicate the relation between the gastrula, blastopore, mouth, and anus; and the site of apical growth. 



nature and of no special phylogenetic significance. The process has nothing in 

 common with gastrulation, and the invaginated cells do not represent a primitive 

 enteron. The cavity of the infolded teloblasts may be called a teloccele, and its 

 external opening the telopore. 



The concrescence that occurs in the craniates is the concrescence of the 

 peripheral margins of an expanding embryonic area, not that of an elongated 

 blastopore. (Fig. 157.) 



FIG. 269. Diagrams of an arthropod embryo. A. B, Sagittal sections; C, D, the embryo seen from the neural 

 surface. The figures indicate the relations between the gastrula, cephalic navel, neurostoma, telopore, and telo- 

 blasts; and the axial structures formed from the latter. Here the gastrula gives rise only to those structures be- 

 longing to the primitive head, or that part of the embryo derived from a coelenterate ancestor. The teloblasts, 

 with or without the formation of a terminal infolding, or telopore, give rise to the axial cords, out of which, like 

 an appendage to the old radially symmetrical head, the new, bilaterally symmetrical, segmented trunk is formed. 



In the craniates, therefore, the gastrula and trochosphere stages and the 

 blastopore are omitted, or are but faintly repeated at the head end of the medul- 

 lary plate, while a conspicuous false gastrulation is produced by the infolding of 

 the teloblastic areas at the end of a rapidly growing trunk. The organs of the 

 trunk, produced by apical growth become recognizable, or separate from each 

 other from before backward, at varying periods in different members of the group, 

 and there is a general tendency to carry the development within the egg up to 

 later and later stages, so that the young, when liberated, usually develop into the 

 adult without a marked metamorphosis. 



