4i8 NATURAL SCIENCE. June, 



it applies to the Protozoa. It is somewhat surprising that little 

 careful investigation of this latter point has as yet been made. 



Secondly, as regards the occurrence of amitosis in the Metazoa. 

 Although a few authorities still doubt its existence, it seems clear that, 

 if we are not to disregard a large number of observations, amitotic 

 division of nuclei does actually take place. This fact was first 

 insisted on by Flemming, and the subject has been carefully worked 

 out by Ziegler and others. These observers have shown, in the first 

 place, that nuclei which divide directly are always of relatively 

 enormous size, a condition probably the result of unusually intense 

 secreting or assimilating processes going on within them. For instance, 

 the ovarian cells of many insects act as conveyers of nutriment to the 

 growing ovum. These cells possess large nuclei, which have been 

 observed to break up directly into two or more pieces, although there 

 is no corresponding division of the body of the cell as a whole. 

 Again, the brood-pouches of certain molluscs are lined by cells which 

 pour into them a nutritive fluid. These cells have large, irregularly- 

 shaped nuclei, which undergo rapid amitotic division preparatory to 

 disintegration. Further, it has been shown that the cells lining the 

 mid-gut of insects have the function of gland-cells. They are con- 

 tinually being given off from the inner surface of the gut, and new cells 

 are formed from underneath to supply the place of the old ones. 

 These young cells are formed, in the first instance, by karyokinetic 

 division, but as soon as they become functionally active, their nuclei 

 swell up and divide directly into several pieces. Soon afterwards the 

 cell dies and is thrown off. To take one more instance, this time 

 from the Mammalia, it has been noticed that amitotic division 

 characterises the nuclei of the epithelial cells in the uterine wall 

 preparatory to parturition. Lastly, amitosis is of frequent occur- 

 rence in the cells of pathological growths of all kinds. In all these 

 examples it must be noticed that, although nuclear division takes 

 place, there is no corresponding division of the cell-body, and, further, 

 that amitosis is a characteristic of nuclei that have taken on a special 

 function, leading to the speedy death and disintegration of the cell. 



It is, however, by no means true that amitosis is never accom- 

 panied by cell-division, or that it is always followed by nuclear 

 degeneration and a stopping of cell multiplication. In the 

 spermatogenesis of many animals, it appears that the sperm- 

 mother-cells divide amitotically up to a certain point, after which 

 karyokinesis marks their further division into spermatoblasts. 

 In the development of the spermatozoa of Branchipus, Moore (14) has 

 shown that some of the primitive genital cells break down to form a 

 fluid, in which lie the spermatozoa produced by the remainder. The 

 former are the product of repeated amitotic division, the latter of 

 karyokinetic. But the more striking instances of fragmentation of 

 the nucleus are met with in the fertilised ova of many animals. 

 Henking, among others, has made careful investigations on insect 



