i2 NATURAL SCIENCE. 



Jan., 



adult or ordinary individuals of a species, from mechanical causes." 

 Conjugation is, however, a necessary part of their life-cycle, and, 

 special individuals, of extremely simple structure — a structure so 

 simple that conjugation between them is possible — are formed. 

 These individuals are the ovum and spermatozoon. A dioecious 

 metazoon is a " tetramorphic species, consisting of male, female, 

 ovum, and spermatozoon." 



New Memoirs on Cells. 



The whole issue of the Quarterly Journal of Microscopical Science, 

 in which occurs Mr. Sedgwick's paper referred to in our last para- 

 graph, consists of memoirs upon cells and nuclei. Mr. J. E. S. Moore 

 writes an extremely interesting memoir upon Elasmobranch spermato- 

 genesis. He divides the spermatogenesis into two periods. The 

 first lasts from the first appearance of the genital cells in the embryo 

 until the actual production of spermatozoa begins. The nuclear 

 divisions in this period are similar to the divisions occurring in the 

 tissues of the body generally. In Scy Ilium canicula, the common dog- 

 fish, when the nucleus is preparing for division, twenty-four chromo- 

 somes appear ; in the division each chromosome splits longitudinally 

 into two. At the end of the first period the cells are lying in the 

 genital ampullae in layers seven or eight cells thick. Avoiding 

 technical details, it is enough to say that marked changes in the 

 structure of the nucleus and the various parts of the cell mark the 

 advent of the second period. When the chromosomes appear for the 

 divisions of the second period, only half the number of those present 

 in the first is found. In the case of the common dog-fish, twenty- 

 four chromosomes appeared in the divisions of the first period; twelve 

 chromosomes appear in the divisions of the second period. These, at 

 first thick loops, become ring-like structures. The actual splitting of 

 the chromosomes takes place across the long axis of the rings, which 

 become pulled out into loops. A second set of divisions, in which 

 also twelve chromosomes appear and divide transversely, gives rise to 

 the actual spermatozoa. Apart from the exact details, the processes 

 described by Mr. Moore have a theoretical interest. As he points 

 out, Weismann laid great stress on the occurrence of a reducing 

 division in the formation of germ-cells, that is to say, of a division in 

 which the chromosomes did not divide, but one-half passed out to 

 each daughter nucleus. Mr. Moore shows that such a reducing 

 division does not occur in the spermatogenesis of the dog-fish : many 

 botanists have shown that there is no evidence for it among plants. 



To the same number Mr. M. D. Hill contributes a valuable 

 memoir upon Fecundation, Maturation, and Fertilisation in Sphcev- 

 echinus and in Phallusia mammillata, and Mr. A. B. Macallum contri- 

 butes a most important memoir upon iron compounds in animal and 

 vegetable cells. 



