CHAP, xiv CHAETOGNATHA 429 



appears at the 8-cell stage, and the result of segmentation is the 

 production of a spherical hollow blastula which, it may be remarked 

 incidentally, is not unlike the blastula of Amphioxus, although at 

 once distinguishable from it by its greater size. One side of this 

 blastula flattens and on the flat side an invagination takes place, and 

 the blastula is thus converted into a gastrula. 



The invagination, which is the rudiment of the archenteron, 

 increases in depth till it practically fills up the entire blastocoele and 

 the endoderm is pressed close against the ectoderm. The whole gastrula 

 then elongates, the blastopore narrows, and the embryo assumes a 

 fusiform shape, being more or less pointed at the anterior and 

 posterior ends and broadest in the middle. 



At the same time the mother cells of the genital organs make 



coe 



FIG. 338. Optical sections of early embryos of Sagitta bipnnctata. (After Doiicaster'. ) 



A, gastrula showing the two mother cells of the genital organs. B, embryo in which the archenteron 

 is being divided by lateral folds into coelomic sacs at the sides and enteron in the middle. Up, blasto- 

 pore ; coe, coelomic sac ; ent, enteron ; </, mother cell of the genital organs. 



their appearance as a pair of large cells which project into the cavity 

 of the archenteron ; they occur in a position about one-fourth the 

 length of the embryo from the anterior end. They are extruded from 

 the wall of the archenteron and are therefore endodermal in origin 

 (Fig. 338). They touch each other and are, of course, situated sym- 

 metrically with regard to the median sagittal plane of the embryo, 

 but, according to Doncaster, they are nearer either the dorsal or the 

 ventral surface of the embryo ; he could not decide which, for he was 

 unable to find any definite landmark to discriminate between dorsal 

 and ventral surfaces. The very early appearance, as peculiar cells, 

 of the rudiments of the genital organs in many groups, such as those 

 treated in this and in the preceding and following chapters, has led 

 to interesting speculations as to whether a reproductive substance is 

 not separated from a somatic substance at the very beginning of 

 development. This problem has been attacked, in the case of the 

 development of Sagitta, by several authors, of whom Buchner has 



