yO MCMURRICH. [Vol. XL 



chorion is a product of the follicle-cells, and is formed within 

 the ovary. 



The ova of Aselhis, Porcellio, and Armadillidium agree per- 

 fectly with those oijaera as regards the presence of mem- 

 branes, the chorion being the only envelope before the extrusion 

 of the polar globules. 



Soon after the ova oijaera reach the brood-pouch, the polar 

 globules are given off, fertilization apparently taking place 

 either in the ovary immediately before the ova leave it or else 

 in the oviduct. How the spermatozoa pass through the cho- 

 rion I could not discover, there being apparently no micropyle. 

 Two polar globules are extruded (Fig. 4, pg), and one or both 

 may subsequently divide, since not unfrequently three globules 

 were visible, two being smaller than the third, and in some 

 cases I observed four. 



During the period occupied by the maturation of the ovum 

 as indicated by the formation of the polar globules a second 

 membrane, the vitelline membrane (Fig. 4, ym) is formed, and 

 its relations to the polar globules possess considerable inter- 

 est. In the majority of cases in which the relations were ob- 

 served both polar globules were between the chorion and the 

 vitelline membrane (Fig. 4), but not infrequently both were 

 within the latter membrane ; and in some cases one was between 

 the two membranes and the other between the vitelline mem- 

 brane and the ovum. 



It seems to be a very general rule that the ova of Crustacea 

 when extruded are enclosed within a single membrane, the 

 chorion, which is almost certainly formed as a secretion by the 

 follicular cells, though certain authors have referred its forma- 

 tion to the walls of the oviduct. A few cases occur in the 

 literature to which I have access which form an exception to 

 this rule. Grobben ('79) states that the egg of Moina possesses 

 no membrane when extruded, but that some time after it 

 reaches the brood cavity one is formed, which, since it must, 

 under the conditions, be a product of the &%g protoplasm, is to 

 be regarded as the vitelline membrane. The same author ('si) 

 has described a similar absence of a chorion in the ova of 

 Cctochilus, and Delia Valle ('89) finds the same conditions in 



