14 SUMMARY OF CURRENT RESEARCHES RELATING TO 



these, four germinal discs were present in the egg, two quite distinct 

 from each other, and two connected. The author believes that this must 

 have been the result of the presence of four germinal vesicles in the egg. 

 The other two eggs were in the gastrula stage, and showed two invagina- 

 tions instead of a single one. From the appearances presented, the 

 author believes that the spreading of the blastoderm over the yolk 

 during the process of invagination arises not only from a direct con- 

 version of the material at the spot into protoplasm, but also by an ex- 

 tension of the already formed blastoderm, due to a spreading-out of 

 elements originally near together. 



Periodicity in Spermatogenesis.* — Dr. Gustave Loisel finds, from a 

 study of spermatogenesis in the sparrow, that periods of spermatogenetic 

 activity alternate with periods of testicular repose. The latter are at 

 first associated with cellular regression and partial or total absorption 

 of the formed elements. As maturity approaches the periods of repose 

 are shorter. It may be, the author suggests, that we have here to seek 

 for an interpretation of sexual precocity and periodicity in mammals and 

 man. 



"Nebenkem" of Sperm-cells.f — Dr. Friedr. Meves has studied, in 

 the male cell of Paludina vivipara and Pygsera bucephala, the structure 

 called by von la Valette St. George the Nebenkem. Meves believes 

 that this term should lapse, and himself employs for the separate granules 

 Benda's term mitocJiondria, and for the Nebenkem which may be 

 formed by their union, the term mitochondrial corpuscle. In Paludina, 

 as is well known, there are two kinds of spermatozoon, the difference 

 being first obvious during the growth-period of the respective spermato- 

 gonia. In the spermatogonia irom which the "hair-like sperms" ulti- 

 mately arise, there are a number of minute scattered mitochondria, 

 which at tie approach of the first maturation division arrange them- 

 selves first in filaments and then in rings. At first small, the rings 

 increase in size, and are as it were pulled out lengthwise, so that they 

 form double threads. At the division equal numbers of those double 

 threads pass into the daughter-cells, and this is repeated at the second 

 division. As the spermatozoon is formed, the threads unite into little 

 vesicles, at one stage four in number, which are the equivalents of 

 the Nebenkem. These vesicles surround the middle-piece of the de- 

 veloping sperm, and by fusion form a complete envelope for it. In the 

 development of the " vermiform sperms " the mitochondria do not fuse, 

 but remain throughout in the granular form. They accumulate about 

 the middle-piece of the developing sperm as granules, and give rise to 

 the appearance of cross-striping. 



In Pygsera bucepliala there are also two kinds of spermatozoon. The 

 development of the sperms is described, but in the absence of figures it 

 may be sufficient to note the chief point of importance — that the mito- 

 chondria after the second maturation division fuse to form a single 

 corpuscle, which is the Nebenkem in von la Valette St. George's sense. 

 In an exhaustive survey of the literature the author points out how 

 these results aid in the interpretation of the more or less isolated 

 observations of other authors on granules and so forth in the sperm. 



* Compt< j 6 Rendus, cxxxi. (1900) pp. 725-7. 



t Arch. Mikr. Auat., lvi. (1900) pp. 553-606 (2 pis. and 2 figs.). 



