DEVELOPMENT. 



405 



C. The Acraniates. — In the acraniates the embryonic development takes 

 place under a new set of conditions and is expressed in a new set of forms. The 

 eggs as a rule are very small, practically devoid of yolk, and develop rapidly, with 

 continuous epithelial layers and folds. The gastrula stages are passed within 

 the egg membranes, and the embryo escapes at an early period as a small unseg- 



FlG. 



-Semi-diagrammatic sagittal sections through the embryo and young larva of Balanoglossus, to illustrate 

 the relation of the neostoma and haamostoma to the gastrula, telocoele and telopore. 



mented larva, or naupula, representing under various disguises the nauplius stage 

 of their crustacean ancestors. For such a large and diversified group of animals, 

 the early embryonic and larval stages are remarkably uniform. The small, more 

 or less transparent eggs undergo a total and nearly equal cleavage, forming a small, 

 hollow blastula (cirripeds excepted). No true gastrulation, such as that in the 



Fig. 271. — Diagrams of the development of a vertebrate and acraniate seen as a semi-transparent object from 

 the neural surface. The primitive gastrula is indicated only by the remnants of the neurostoma, now a shallow pit 

 lying on the floor of the procephalic lobes, and later, after the closure of the medullary plate, giving rise to tho 

 infundibulum and the saccus vasculosus. The teloblasts have increased greatly in importance, and, like those in 

 the arthropods, give rise to a large terminal infolding, or false gastrula, from the walls of which the axial cords, such 

 as the notochord, mesoderm and endoderm are formed. These structures separate at various periods, and in 

 various manners, as shown by the cross-sections E-H, but in all cases the end-result is the same, and the real 

 sources of the axial cords are special groups of proliferating cells lying at the caudal apex of the trunk. 



annelids, occurs in the group, but there is a large infolding, or mesentocoele, at 

 the posterior end of the blastula, in which the teloblasts and their earlier products 

 are involved. (Fig. 271, yl.) It opens outward by a telopore that marks the 

 caudal end of the body, and closes near the point where the anus is formed. It 

 never remains open as the primitive mouth, and is never formed in the oral or 

 cephalic region. 



The component parts of the infolded layer begin to separate, or first become 



