STRUCTURE OF THE OVUM IN FISHES AND REPTILES. 185 



extremity of the epithelial cells which is situated nearer to the 

 follicular wall, new protoplasm is constantly being formed, and 

 forced onwards. However this may be, two circumstances 

 show that the follicular epithelium stands in immediate rela- 

 tion to the formation of the secondary yolk. In the first place, 

 very small bright structures, resembling the youngest vitelline 

 granules, are already found, especially in Lizards, in the proto- 

 plasm of the epithelial cells, and secondly, the epithelial cells 

 of the follicle (fig. 194) are most strongly developed during 

 the period that the formation of the yolk is taking place with 

 the greatest activity. Meckel, v. Hemsbach (73), Allen Thom- 

 son (115), and others, have taken the same view. The radially 

 striated layer, situated (fig. 194) between the follicular epi- 

 thelium and the yolk, can form no obstacle to this mode of 

 formation, since, as I have already shown, it is perforated with 

 numerous pores, in which fine fibres of the protoplasm of the 

 epithelial cells are lodged. 



The ova of the Selachia exhibit very similar characters, as has 

 been especially demonstrated by the researches of Gegenbaur 

 (43). I find also that this may be said in regard to the formation 

 of the ova in the Osseous Fishes and the higher Crustacea, as, for 

 example, the Astacus fluviatilis, in which the vitelline lamellae 

 and vitelline spheres originate in exactly the same mode, and 

 in the ova of which a principal and secondary yolk, like 

 those of the Bird, may be clearly distinguished. As showing 

 the complete and thorough difference existing between the 

 principal and the secondary yolk, Strieker (114) has made 

 an interesting observation in the ovum of Forella, in which, 

 under favourable circumstances, amoeboid movements can be 

 observed in the former, whilst the latter remains perfectly 

 passive. 



The ova of the Batrachia, though they otherwise closely 

 resemble those of the Osseous Fishes, do not permit a clear 

 distinction to be perceived between the principal and the 

 secondary yolk ; in this respect they rather resemble those of 

 Mammals. Still many circumstances, as, for example, the 

 presence of vitelline lamellae, which are quite similar to those 

 of fishes, show that here also a secondary vitelline mass is 

 formed from the follicular epithelium, which cannot so easily 



