THE OVUM. 51 



less membrane of the ovarian wall. The stalks of projections 

 so formed are turned towards the lumen of the ovary, and are 

 plugged with the epithelial cells which line the ovarian sack. 

 When ripe, the ova pass from their sacks into the cavity of the 

 ovary. The yolk-nucleus, which appears very early, is a solid 

 body present in the protoplasm of the ovum. It is not found in 

 all genera of Araneina. At its full development it exhibits in 

 the fresh condition a granular structure, but very soon shews an 

 irregularly concentric stratification which becomes more marked 

 on the addition of reagents. According to Balbiani this strati- 

 fication is confined to the superficial layers, while internally there 

 is a body with all the characters of a cell. The yolk-nucleus is 

 still found in the nearly ripe ovum, though it always disappears 

 before development commences. It is probably connected with 

 the nutrition of the ovum, though nothing is certainly known 

 about its function. 



CRUSTACEA. 



(57) Aug. Weismann. " Ueb. d. Bildung von Wintereiern bei Leptodora 

 hyalina." Zeit.f. wt'ss. Zoo!., Vol. xxvii. 1876. 



[For general literature vide Ludwig No. 4 and Ed. van Beneden, No. 1.] 



Amongst the many interesting observations on the Crustacean 

 ova I will only allude to those of Weismann on the ova of Lepto- 

 dora, a well-known Cladoceran form. 



The phenomena of the development of the ova in this form 

 present a close analogy with those in Insects. 



The ovary is formed of (i) a germogen containing at its upper 

 end nucleated protoplasm and lower down germinal cells in 

 groups of four ; (2) of a portion formed of successive chambers 

 in each of which there is a row of four germinal cells. Of the 

 four cells only the third develops into an ovum ; the remainder 

 are used as pabulum. This is the mode of development in the 

 summer. In the winter the sacrifice of a larger number of germi- 

 nal cells is required for the development of the ova ; and an 

 ovum is produced only in the alternate chambers. In the 

 chambers where an ovum will not be formed an epithelial invest- 

 ment becomes first established round the four germinal cells. 

 The four cells then coalesce, and form a spherical ball of proto- 

 plasm from which portions are budded off and absorbed by the 



42 



