ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 655 



of the changes, transverse segmentation not appearing till later (diplo- 

 tamie stage). 



In regard to the organogenesis of the ovary, the author finds that tho 

 primordial germinal epithelium gives origin successively to the follow- 

 ing structures : medullary cords, true germinal epithelium, and epithelial 

 invaginations. 



Zona pellucida and Egg.* — V. von Ebner notes that most modern 

 anatomists believe that Flemming, Paladino, and Eetzius have proved that 

 the radial striping of the zona pellucida is due to prolongations from 

 the follicular cells which pass into the egg, and that the egg grows while 

 enveloped in the membrane. Nagel, on the other hand, believes that a 

 perivitelline cleft exists between the surface of the egg and the inner 

 surface of the zona, and that the latter does not appear till the egg has 

 attained its full size. The author believes that Nagel's " perivitelline 

 cleft " is an optical effect in the case of fresh eggs, though such a 

 space may exist in degenerating eggs. Further, very careful measure- 

 ments have convinced him that the egg does increase in size while it is 

 invested by the zona. As the zona is a formation which arises between 

 the intercellular bridges connecting egg and follicular cells, there is 

 no difficulty in supposing that it can increase in diameter, and so allow 

 room for the increasing egg. 



Effect of Gravity on Development, f — Prof. Oskar Schultze notes 

 that the dispute between himself and Roux on this point suffers from a 

 want of precision in the use of terms. The problem which he has set 

 before himself is as follows : — Is it necessary for the normal development 

 of the frog's egg that its chief axis should be allowed to take up a 

 definite position under the action of the force of gravity ? In such a 

 form the problem is capable of definite experimental investigation. In 

 the first place, while within the ovary the egg does not appear to be 

 influenced by gravity, and this peculiarity is retained until fertilisation. 

 Alter fertilisation the secretion of the perivitelline fluid makes it possible 

 for the egg to rotate in its membranes, and so to respond to the action 

 of gravity by the adoption of one definite position. If this be rendered 

 impossible, the egg perishes ; if it be prevented for a limited period and 

 then gravity be permitted to again exert its effect, abnormalities of 

 varying degree are produced. Much of the paper is concerned with a 

 detailed discussion of the methods of experiment adopted by Roux and 

 the author respectively, and a criticism of Roux' results. 



Development of Spleen4 — Dr. W. Tonkoff has studied this subject 

 in the case of the Amniota, and finds that the spleen originates in them 

 from cells split off from the ccelomic epithelium. These at once lose 

 all connection with the epithelium, and become indistinguishable from 

 the mesenchyme. The connective-tissue basis of the pancreas and the 

 investment of the duodenum originate in precisely similar fashion, save 

 that in their case the cell-migration takes place even more rapidly. If, 

 therefore, these are to be reckoned as mesenchymatous, then the spleen 

 is mesenchymatous. Its origin should be contrasted with that of such 



* Anat. Auzeig., xviii. (1900) pp. J5-62 (2 figs.). 



t Arch. Mikr. Anat., lvi. (1900) pp. 309-34. Cf. this Journal, 1S95, p. 292. 



j Tom. cit., pp. 392-458 (3 pis. and S ligs.). 



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