OVULATION IN OSSEOUS FISHES AND BATRACHIANS. 595 



detached from the ectosac, as it becomes after impregnation. 

 Against the ectosac, h, are closely applied the flattened epithelial 

 cells, d, which line the ovisac and probably contribute to the 

 growth or thickening of the external membrane. In the Perch 

 the ectosaccal tubes have a 



slightly spiral course, and 416 



the peripheral end of each 

 is set in a small hexagonal 

 prism : this outer stratum 

 serves to connect the ex- 

 cluded ova in groups. The 

 ova of the Stickleback have 

 villi developed for this pur- 

 pose from one part of the 

 exterior, figs. 417 and 421. 



The structure of the outer 

 coat of the ova of Fishes re- 

 lates not only to the accidents 

 of appulse, but to the act of 

 impregnation, which takes 

 place after exclusion and 

 exposure. Besides the pores, 

 or tubular orifices, of the 

 ectosac, there has been observed, in the ova of Gasterosteus, 1 

 Salmo, 2 Blermius, 3 Esox, Trigla, A a foramen homologous with the 

 micropyle of the invertebrate ovum. Dr. Ransom, its discoverer, 

 has observed the passage of the spermatozoa through this foramen 

 into the ovum. Fig. 417, A, gives a magnified view of a portion 

 of the surface of a mature ovarian ovum of the Stickleback, 

 showing the funnel-shaped depression leading to the aperture of 

 the micropyle, with a few of the flask-shaped villi of the outer 

 membrane : b represents diagrammatically a section of the ectosac 

 and funnel of the micropyle : c is a portion of the membrane 

 at the apex of the funnel, with the aperture of the micropyle 

 pressed flat, of the Trout's egg, magnified 500 diameters : d is 

 a similar portion, magnified 1 ,000 diameters, showing the l lumen ' 

 of the ectosaccal tubules, and the hexagonal division of the spaces 

 between them. 5 



In the Batrachia (Frog) a greater proportion of the ovarian ova 

 show different states of developement, previous to sexual pairing, 

 than in Osseous Fishes, although all those destined for impreg- 



Tart of tho ovarian ovum of the Salmon. CCOVIII. 



1 CLXXVI. 



2 cccvni. and cccix. p. (101). 



cxxx. pt. ii. p. 4. 



cccx. p. 37G, %. C. 



cccvin. p. (101), 



