ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 177 



beginnings in the tubules of the mesonephros. A detailed paper is 

 promised. 



Pelvic Plexus of Mustelus.* — Mr. E. C. Punnett has studied embryos 

 of M. leevis and M. vulgaris, with special reference to the question of the 

 ontogenetic history of the nervus collector, and a brief abstract of his 

 paper is published. He finds that in the embryo there is a posterior col- 

 lector, from which in the later stages the component nerves separate to 

 run singly into the fin. This goes to prove that the collector condition 

 is primitive. It has apparently arisen as the result of the rostral migra- 

 tion of the whole fin. 



Ovarian Follicles in Cymatogaster.t — G. L. Mitchell considers that 

 the structure of the follicles in this viviparous fish sheds light on the 

 origin of the Graafian follicles of the mammalia. The eggs of the fish 

 are very small, most of the yolk having been lost, and in the ovary, be- 

 sides the normal single-layered follicles, aberrant many-layered follicles 

 also occur. These resemble early stages in the development of the 

 follicles of Metatheria and Eutheria, and are regarded by the author as 

 the result of the reduction in size of the egg. The condition is thus 

 analogous to that which occurs in the Metatheria and Eutheria, where 

 the ovum has been reduced in size as compared with the ova of Mono- 

 tremes and Eeptiles. In the latter the follicle is single-layered, just as 

 it is in the normal follicles of Cymatogaster, and in each case the reduc- 

 tion of the egg seems to result in an increase of follicular cells. 



Development of Urino-genital Organs in Petromyzon. J — Prof. 

 W. M. Wheeler now publishes the results of observations made in 1893 

 on this subject, together with a historical survey, and a general discussion 

 of the urino-genital system of the Anamnia, its origin, and the relations 

 of its elements. He finds that in the lamprey the pronephric duct arises 

 by the fusion of abortive pronephric tubules. These abortive tubules 

 have not grown backwards from the functional pronephros, but originate 

 in the region in which they remain. This the author regards as the 

 primitive condition, a view for which he finds confirmation in isolated 

 observations of other investigators for different Anamnia. As the meso- 

 nephric tubules open into the pronephric duct, or in other words, as 

 both sets of tubules coexist in the functional condition in the same 

 segments, there can be no question of serial homology between the 

 two. 



While in Myxine the mesonephros is of very simple structure, in 

 Petromyzon it displays considerable complexity. A special peculiarity 

 is that it shows no trace of metamerism, its tubules being always more 

 numerous than the body-segments. This is to be ascribed to a hastening 

 of events ; for the author believes that it is in Selachians that the primi- 

 tive condition of the mesonephros is to be sought, and that the conditions 

 in Teleosts and Petromyzon are derivative. Apart from the absence of 

 metamerism already mentioned, the most striking peculiarity of the 

 mesonephric tubules in Petromyzon is their origin from solid peritoneal 



* Proc. Pov. Soc. London, Ixv. (1900) pp. 445-6. 



t Proc. In.i'iaua Acad. Sci., 1S98, pp. 229-32 (7 figs.). 



J Zool. Jahrb. (Abt. Anat.), xiii. (1899) pp. 1-SS (7 pis.). 



