Mitocho7idria, Golgi Apparatus, and Yolk. 137 



which demonstrate the mitochondria of the sponge Q^g, bring into 

 (so called chromidia) evidence the yolk spheres. In no form could 

 one feel more certain that the yolk spheres are not metamorphosed 

 mitochondria. The yolk granules of Grantia are formed in and 

 by the ground cytoplasm, aud are of a much more delicate nature 

 than the true mitochondria. The latter, in sponge eggs, are so 

 large and so few, that there can be no question of confusion of 

 yolk spheres and mitochondrial spheres at any stage of oogenesis. 



So far as could be ascertained the yolk spheres of the Grantia 

 egg are of a semi-fluid consistency, and possibly contain a small 

 amount of protein, but a large bulk of lipin. The spheres are 

 believed to arise as small vacuoles in the ground cytoplasm of the 

 endoplasm of the egg. Techniques which depend for their efficiency 

 on the coagulation of protein are quite inadequate for the preserva- 

 tion of the sponge yolk, while methods known to fix fat or lipins 

 are the only ones suitable for the yolk of the Gra7itia oocyte. The 

 spheres in the Grantia oocyte do not go black in osmic acid, even 

 after prolonged immersion in strong solutions. 



Oogenesis of Mollusca (Helix and Llmn^ea). 



As a simple type the oogenesis of a mollusc {Limnsea) may be 

 taken. Lately it has been shown {16) that the ripe egg contains 

 three categories of granular or formed structures, viz. mitochondria,. 

 Golgi elements and yolk spherules. The mitochondria and Golgi 

 elements appear to be present in the germinal epithelial cells, 

 though it has been difficult to make certain with regard to the 

 mitochondria ; it is however a fact that in Amphibia {Bano, BufOy 

 Molge), Birds (Gallus), Mammals (Mus, Lepus), in other Mollusca 

 {Helix, Arion), in Insecta {Apanteles, Sphinx), the primitive 

 germinal cells contain mitochondria and Golgi apparatus. 



It has been shown that in such a mollusc as Helix or Limnsea 

 the mitochondria and Golgi elements gradually spread out through- 

 out the oocyte, and the grains forming these systems increase in 

 number. It seems quite probable that the diffuse Golgi elements 

 actively take part in the formation of yolk bodies, as indicated 

 in PL II, fig. 5 ; we cannot say so much in the case of the mito- 

 chondria. From a study of a number of Pulmonate Mollusca we 

 have concluded that much of the evidence in these forms is against 

 the view that part of the mitochondrial constituents of the cyto- 

 plasm metamorphose into yolk. The latter seems to form either 

 from Golgi elements, or per se in the ground cytoplasm. Since we 

 cannot yet give a definite opinion on this point, we have taken 

 care to leave the matter open in our schemes of oogenesis given on 

 page 148 of this paper. 



