384 L. DONCASTKR. 



Between the epithelial layer which lies against the body- wall 

 and the layer of coluniuar germinal cells there appears at 

 this stage^ at the base of the ovary, a mass of loose tissue 

 with round nuclei {od. c). A rather later stage shows these 

 cells collecting themselves into a double layer, forming the 

 lining epithelium of the oviduct, which, however, up to this 

 point contains no lumen. The oviduct shortly before nuiturity 

 has two walls, — an inner of cells with round nuclei which as 

 yet enclose no cavity, and an outer which on the inner side 

 forms the germinal epithelium {germ, ej).), while on the 

 outside it consists of cubical cells constituting the limiting 

 epithelium above described (tig. 39). The oviduct thus lies 

 wholly within the ovary, except at the point at which it opens 

 to the exterior, where it becomes continuous with the epi- 

 dermis. There is, however, never any trace of invagination 

 of the epidermis during the growth of the ovary, but as the 

 latter grows forward the loose tissue above described {od. c.) 

 grows with it near its outer border, and when the ova become 

 nearly mature gives rise to the oviduct which runs along the 

 whole length. The development of the oviduct is very rapid 

 and takes place just before maturity, and from the facts 

 described it seems certain that it is developed from the ovary 

 itself, and not by an invagination of the body-wall as was 

 suggested by Hertwig, for no trace of such an invagination 

 has ever been seen. 



VIII. Summary and Conclusions. 



The development of the genus Sagitta may be summarised 

 in its most important facts as follows : 



The cleavage is complete and equal, giving rise to a spherical 

 blastula of apparently similar cells, the nuclei of which lie at 

 their outer ends, and from this a gastrula arises by invagina- 

 tion. When the gastrula is well formed two cells are separated 

 off from its inner layer at the opposite pole from the blasto- 

 pore j these soon divide into four, which constitute the prniitive 

 genital ceils, remaining unaltered until a late stage of de- 



