ON THE DEVELOPMENT OF SAGITTA. 383 



body-cavity and there develop furtlier, as described by 

 Grassi, and at about the same time the male genital ducts 

 begin to arise. In their first origin the latter appear as a 

 thickening of the ectoderm in the lateral line in the space 

 between the paired fin and the tail fin, just at the hind end 

 of the former. The thickening increases and soon splits into 

 two well-defined layers, and then the upper layer separates 

 from the lower like a blister, leaving a space between them 

 (PL 21, fig. 36, V. s.). Traced forward this space is found to 

 narrow into a very fine canal, enclosed in a few well-defined 

 cells and lying between the ectoderm and the mesoderm, but 

 from its continuity with the larger space behind there can be 

 no doubt that the whole is ectodermal in origin. This canal 

 runs forward for a considerable distance, and its walls then 

 join the lining of the ccelom into which the canal opens 

 (fig. 37, V. d.). As the animal nears maturity the larger 

 space behind increases in size, and at its front end a longi- 

 tudinal groove appears in the outer wall, along which the 

 opening to the exterior is formed. The whole space forms 

 the seminal vesicle of the adult, and the chitiuous '' calotte " 

 is formed only at maturity. 



In the female organs the course of development is different. 

 After the stage is reached in which the rudiment of the ovary 

 is a mass of similar cells, this mass grows forward and 

 becomes more cylindrical in shape. A differentiation of the 

 cells then begins to take place, those next to the body-wall 

 becoming a sort of epithelium, while those towards the 

 body-cavity become the primitive ova (fig. 38). The latter 

 have two forms ; those in the centre of the ovary become 

 columnar, with elongated nuclei, and give rise to the genital 

 epithelium, the cells of which (or some of them) later enlarge 

 and become ova, as described by Grassi. Between this layer 

 and the inner edge of the ovary, already in quite immature 

 individuals larger rounded cells are found, Avhich will be the 

 first ova to mature. Only parts of two or three of the latter 

 appear in fig. 38 (o.) because the section is taken close to the 

 base of the ovary, and they occur chiefly near its free end. 



