OVULATION IN OSSEOUS FISHES AND BATRACHIANS. 



416 



detached from the cctosac, as it becomes after impregnation. 

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

 cells, (/, which line the ovisac and probably contribute to the 

 growth or thickening of the external membrane. lu the Perch 

 the ectosaccal tubes have a 

 slightly spiral course, and 

 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 

 imj^regnation, which takes 

 place after exclusion and 

 exposure. Besides the pores, 

 or tubular orifices, of the 



ectosac, there has been observed, in the ova of G aster osteus,^ 

 Sulmo^ Blennius,^ Esox, Trigla,^ a foramen homologous with the 

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

 has observed the passage of the sj^ermatozoa 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 funuel-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 microj)ylu 

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

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

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

 between them.^ 



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- 



' OLxxvi. - cccvni. and cccix;. p- (101). ' cxxx. pt. ii. p. 4. 



* cccx p. 37G, fi^'. 6. * cccviii. p. (101), 



QQ2 



Part o£ Uic ovarian ovum of tbo Salmon, cccvui, 



