356 SUMMARY OF CURRENT RESEARCHES RELATING TO 



MoUuscoida, 



Development of Ova in Ascidians.* — A. Sabatier, who Las investi- 

 gated a large number of Ascidiaus, finds that: — 



The ovary is composed at first of nuclei arising from the mesoderm 

 and connected together by a small amount of clear intermediate sub- 

 stances. This structure is seen in the adult in those regions of the 

 ovary in which there is a fresh formation of ova. The ovum arises 

 primitively from one of the corpuscles of this tissue, and this, in 

 which one or two granulations appear to form nucleoli, constitutes the 

 nucleus of the future egg. Around this a layer of transparent colour- 

 less protojjlasm becomes arranged, and thus the essential elements of 

 the egg are connected together. A delicate membrane, which appears 

 to arise from the intermediate substance of the embryonic connective 

 tissue of the ovary, forms a capsule for the egg. Below this and on 

 the surface of the yolk, cellular elements arise ; these are formed in 

 the yolk and not from outside it. Below them and at their expense 

 is developed a second vitelline membrane. The so-called granular 

 cells are, as Kuppfer and Semper have taught, likewise of vitelline 

 origin ; the intra-vitelline corpuscles are also not immigrated bodies, 

 but they owe their origin to a concentration of granules within the 

 yolk itself. 



Structure of Ovary of Phallusiadse.t — L. Eoule, after stating 

 that he is able to confirm the accounts given by Fol and Sabatier of 

 the ova of Ciona intestinalis, states that the ovules ai'e derived from 

 endothelial cells, which have a protoplasmic layer around their 

 niicleus and an external enveloping membrane ; this last becomes the 

 very delicate vitelline membrane, while the protoplasm and nucleus, 

 increasing in size, form the yolk and the germinal vesicle. The ovules 

 may be found at all stages of development and appear to radiate round 

 a centre of formation. We may then see that the germinal vesicle of 

 young ova contains, in addition to tlie large nucleolus, two or three 

 smaller nucleoli ; in larger eggs there are five or six ; gradually these 

 approach the periphery, and appear to pass to the exterior. Those 

 that pass into the yolk may be further traced as cells of the egg- 

 covering, and the testa thus formed may be looked upon as a remnant 

 of the ovular excretion which has produced the follicle ; in some cases 

 the last formed " nuclei " are sufficiently numerous to form a complete 

 layer of the testa, while in others they only produce separate masses. 

 This phenomenon is the cause of variation in the appearance of the 

 ova of different species of Ascidians, and in some cases of different 

 individuals. 



Embryonic Development of Salpidae.t— W. Salensky finds that 

 in the earliest stages the processes of maturation of the ovum of Salpa 

 consists in the shortening of the ovarian pedicle, and in the formation 

 of polar cells which appear before the process of shortening is com- 



* Comptps Eendus, xcvi. (1883) pp. 799-801. 



t Ibid., (1883) pp. 1069-72. 



X MT. Zool. fStat. Neapel, iv. (1882) pp. 90-171 (12 pis.). 



