36 THE STRUCTURE OF CELLS 



And since, during the reduction division, there is no evidence of 

 the elimination or degeneration of any chromosomes, it is further 

 urged that each of the apparent units appearing in the prophase 

 of the heterotype division are really bivalent, and represent two 

 chromosomes joined end to end, but otherwise behaving as one. 

 Hacker, who has ably supported this view, believes, in common 

 with many who share it with him, that the tetrad is to be thus 

 explained. The longitudinal fission which divides the bivalent 

 rudiment of the chromosome is succeeded by a more or less trans- 

 verse separation or isolation of the chromatin, which marks the 

 fourfold character of these chromosomes. Consequently each 

 tetrad really represents not one but two chromosomes, and whilst 

 the first (heterotype) division corresponds to the line of longitudinal 

 cleavage, the second (homotype) separates and distributes actual 

 entire chromosomes. Hence it is supposed that a real distribution 

 of entire and diverse chromosomes occurs at the homotype mitosis, 

 whilst the heterotype is essentially similar to a somatic division, 

 and the reduction in number (due to the coherence in pairs) of the 

 chromosomes is only an apparent one. If it could be universally 

 proved to be true, such an explanation would account for many of 

 the remarkable peculiarities which, as has been seen, characterise 

 these divisions, besides affording a very strong support to the 

 theoretical views as to the nature of the mechanism of inheritance 

 advanced by Weismann. But the apparently well-established belief 

 that in other cases the preparation for the two divisions is accom- 

 plished by means of two longitudinal divisions of the chromatic liniri 

 militates strongly against conceding the value of a general inter- 

 pretation to the views just sketched in outline. And moreover the 

 facts of amitosis as known to occur in some instances, also, though 

 in a somewhat different way, tell against the permanence of the 

 chromosomes, and consequently against the theories which have 

 been founded on that hypothesis. On the whole, the facts at present 

 before us rather tend to support the view of the brothers Hertwig ; 

 according to them the real significance of the process lies in that 

 sudden quantitative reduction of the chromatin which is a necessary 

 consequence of the rapid succession of the two mitoses in question. 



It has already been pointed out that the reduction divisions are 

 a common feature to both animals and plants. In the latter, how- 

 ever, there appears to exist a much greater latitude as to the point 

 in the life- history at which they occur. In all the archegoniate 

 series of cryptogams, which includes the mosses, hepaticae, and 

 vascular cryptogams, as well as in all the flowering plants, the re- 

 duction divisions are not immediately connected, as they are in 

 animals, with the formation of the sexual cells, but with the asexual 

 spores from which the generation bearing the sexual organs arises. 

 Thus, after the homotype (or second) division, an indeterminate 



