328 SUMMARY OF CURRENT RESEARCHES RELATING TO 



hsemolympli of Planorbis, like that of Vermes, does not coagulate 

 spontaueoush^ ; the coagulation temperature is very different to that 

 of the haemolymph of the Gastropoda, for while this coagulates at 

 60° C, a small amount of fluid can be filtered from the former at 

 64° C. The coloration of the fluid of Planorlis is solely due to its 

 haemoglobin, but the intensity of the colour is never so marked as it 

 is generally in the Mammalia. 



Mollusca. 



Development of the Cephalopoda.*— Dr. M. Ussow, in describing 

 the formation of the germinal glands, points out that the unpaired 

 ovary is a conical sac occupying the lower part of the trunk, and often, 

 when mature, of considerable size. The ripe ova fall into the coelom, 

 and thence by the ciliated epithelium are carried to the oviduct. By 

 the antiperistaltic movements of these latter, they are conveyed into 

 the respiratory cavity, and thence by the contraction of the funnel to 

 the exterior. The Graafian follicles are so arranged that the central 

 portion of the ovary is occupied with the younger or with the primor- 

 dial ova. Each follicle has a separate theca, which is well provided 

 with blood-vessels coming from the genital arteries. The first rudi- 

 ments of the germinal glands appear during the periods of embryonic 

 development, the small group of rounded mesodermal cells which 

 appear in the third developmental period near the narrow end of the 

 mantle and behind the systemic hearts, being, undoubtedly, converted 

 into ovarian glands or sperm-glands. Fm-ther development, and the 

 formation of the efferent ducts appear to be post-embryonic. During 

 these changes the mesodermal cells become converted into a number 

 of racemose Graafian follicles, the walls of w'hich are formed by the 

 thin theca, and by a uni- or bilaminate memhrana granulosa. A pri- 

 mordial ovum and the formative yolk are nothing more than a differ- 

 entiated and greatly developed epithelial cell of the ovary. As the 

 cell grows the Graafian follicles increase in size ; folds then appear 

 owing to the development of the gramdosa-cells, their glandular inner 

 surface increases, and secretes the nutrient fluids. The chorion is 

 not foi-med till the secretion of the yolk is completed, and when it is 

 formed there appears the micropyle ; the chorion is elastic and trans- 

 parent. Beneath it in the mature egg there is an inconsiderable 

 quantity of fluid, which coagulates on heating, and within this there 

 is the formative yolk, formed of a finely granular protoplasm and 

 investing the less fluid nutrient yolk. 



The first developmental period extends from the commencement 

 of segmentation to the first api:»earance of the rudiments of the organs ; 

 there appears to be a striking similarity in the phenomena exhibited 

 by different members of the group. At first all the cleavage-cells 

 appear at one pole of the egg, the grooves extending from the central 

 portion of the formative yolk outwards : the nutrient yolk is regarded 

 by the author, in opposition to Prof. Lankester, as playing a merely 

 passive part. Cleavage is at first superficial and only gradually 

 extends to the more deeply lying parts ; in Argonauia argo there was 



* Arch, cle Biol., ii. (1881) pp. 553-635 (2 pis.). 



