90 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



cytoplasm seems to be attached to the hyaline membrane (mh. prp.) 

 surrounding the whole structure. This membrane is nothing but 

 a local differentiation of a similar covering of the ovary and all its 

 products, the consideration of which has been postponed until now. 



Distaplia seems to have a special aptitude for the secretion of struct- 

 ureless membranes, for they cover the surfaces of nearly all the 

 visceral organs. In the case of the ovary and its products, the mem- 

 brane can usually be demonstrated with ease. Both the connective- 

 tissue cells and the epithelium of the ovary itself seem to participate 

 in the secretion. Origin from the first source is to be inferred both 

 from the frequency with which strands of connective tissue are seen to 

 join and extend along the wall of the ovary (Plate 1, Fig. 6, fun. 

 corCt. tis.) ', and also from the fact that often the connective tissue cells 

 themselves are found on the wall of the ovary. That the membrane is 

 also in part secreted by the cells of the ovary is proved by its general 

 occurrence, but principally by the fact that on the periphery of the 

 stalk tissue, where we have the greatest number of cells per unit of 

 area, it is exceptionally developed, being much thicker than at other 

 places (Fig. 5, mb. prp.). It occurs also on the surface of the follicles, 

 varying much in thickness, but being in general best developed on 

 those that are about half grown. On the surface of the oldest follicles 

 are found here and there very flat nuclei that are probably derived from 

 wandering cells almost imbedded in this membrane. It is probably 

 nuclei of this kind, and the structureless membrane containing them, 

 that Julin considers to be the remnant of the limiting epithelium of 

 the ovary surrounding the follicle ; for he says that this limiting epithe- 

 lium has gradually become hyaline. 



On the newly formed corpus luteum this basement membrane (Plate 

 6, Fig. 47, mb. prp.) is thicker and more conspicuous than on any 

 other part of the ovary, except some of the stalk tissue, and it is of 

 special importance in attempting to explain how the ovum is pressed 

 through the ovary into the oviduct. This pressure cannot be due to 

 muscles, as there are none in the abdomen except those of the heart. 

 Nor can an increase of blood pressure in the whole abdomen, due 

 either to long-continued beating of the heart in one direction, or to 

 the contraction of thoracic muscles, be invoked in explanation. For 

 while this might burst the follicle by pressing it against the ectoderm 

 and modifying its spherical shape, it would also tend to press the 

 oviduct out flat against the ectoderm, and so could hardly be effective 

 in moving the ovum into the oviduct. The only other available force 



