THE OVUM. 41 



wards 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 Balbiaui this stratification 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 WintereiernbeiLeptodorahyaliua." 

 Zeit.f. wiss. ZooL, Vol. xxvu. 1876. 



[For general literature vide LuclwigNo. 4 and Ed. Van Beneden, No. i.] 



Amongst the many interesting observations on the Crustacean 

 ova I will only allude to those of Weismanu on the ova of Leptodora, 

 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 (1) 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 germinal 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 investment becomes first established round the 

 four germinal cells. The four cells then coalesce, and form a 

 spherical ball of protoplasm from which portions are budded off and 

 absorbed by the investing epithelial cells, which at the same time 

 lose their nuclei. When the whole of the central ball is thus 

 absorbed by the epithelial cells, the latter become used by the winter 

 ovum as food. The winter ovum at its full development is formed of 

 a central mass of food-yolk and superficial layer of protoplasm. 



CHORDATA. 



Urochorda. (Tunicata). 



(58) A. Kowalevsky. "Weitere Studien ii. d. Entwicklungd. Ascidien." Archw 

 f. micr. Anat., Vol. vii. 1871. 



(59) A. Kowalevsky. "Ueber Entwicklungsgeschichte d. Pyrosoma." Arch. f. 

 micr. Anat., Vol. xi. 1875. 



