The Spermatogunesis iu Peutatoma up to the Formation of the Spermatid, ß9 



in this inquiry, for two reasons : 1) because iu them occur the pheno- 

 mena to be explained; and 2) because in them frequently, if not al- 

 ways, are found marked deviations from normal (more primitive) mitosis. 

 The more important of these deviations are: the occurrence of two 

 mitoses without an intervening rest stage, or with only the commence- 

 ment (anaphase) of a rest stage ; the frequent number of half of the 

 normal number of chromatin elements in the prophase of the 1st re- 

 duction division; and the precocious segmentation or splitting of the 

 chromatin elements, which in normal mitoses typically takes place at 

 metakinesis. Minor differences may also be noted : thus the chromatin 

 elements of the reduction divisions have frequently a form different 

 from those in the other mitoses of the body. Then, iu ovogenesis 

 (the mitoses of which are in many respects more atypical tliau those 

 of spermatogenesis), we frequently find the absence of pole fibres (and 

 centrosomes ?) in the reduction spindles: they may be absent in both 

 spindles {Ascaris), or may be present in the first and absent in the 

 second (as I have found in the guinea pig); and in ovogenesis also 

 may be noted the inequality in the size of the cells produced by the 

 reduction divisions. With these facts in mind, it seems very justifiable 

 to conclude, that in most cases the reduction mitoses appear second- 

 arily modified, a view which seems to be generally held, though not 

 universally. 



What must be taken as representing more primitive mitoses, and 

 hence as the starting point for the solution of our question, are the 

 divisions of the spermatogonia and ovogonia and of the adult tissue 

 cells. I emphasize the limitation "adult" for the latter, since there is 

 at least one good case known (Ascaris) where the number and form 

 of the chromatin elements in the cleavage cells (from about the 4-cell 

 stage on) is very different from that found in cells of more mature 

 tissues. These strange relations were discovered for Ascaris by 

 BovEEi ('87, '92 a, '92 b), and subsequently corroborated by Herla 

 ('93), Meyek, Zoja ('96) and Zur Strassen. In the cleavage cells a 

 large number of chromosomes occur, some 60 or more ; and in mitosis 

 there is a successive discharge of them out of the equatorial plate. 

 What the significance of these phenomena is, has not yet been fully 

 determined, though the study of them bids fair to promise as much 

 interest as the investigation of the reduction divisions themselves. The 

 early embryonic mitoses — iu Ascaris at least — cannot be considered 

 primitive, the loss of chromatin being a phenomenon which does not 

 take place in ordinary cell division. 



