REPRODUCTION 



421 



In this figure it can be seen that, while the long chromosomes of the 

 upper cell in A divide, as is usual, by a complete splitting into equal 

 halves, in B the chromosomes become separated by transverse breaks 

 into many parts, and that when the division occurs, only the smaller 

 middle parts are drawn apart to form the daughter nuclei; the longer 

 end-portions being allowed to remain and become an inert part of the 

 cytoplasm. In other words, one has become differentiated probably by 

 the loss of some mechanism of use only to a reproductive cell. It has 

 retained every feature but that one and possesses everything necessary 

 to become any kind of a body cell or somatic cell. It has lost something 

 that can never be replaced and it is destined to run a course of develop- 

 ment and differentiation that will end in death for all its descendants. 

 (Read Chapter IV for certain other developments of this idea.) 



FIG. 380. Bit of germinal ridge of a young Acanthias embryo (10 c.m.)- Shows three large 

 reproductive cells in the germinal epithelium, x 1230. 



For a number of subsequent divisions the reproductive cell continues 

 to give off one somatic cell at each division, while the first somatic cell 

 can produce nothing but somatic cells. After this the reproductive cell 

 disclaims all responsibility, so to speak, for the soma or body, and de- 

 votes its time to self-multiplication and the production of a few accessory 

 cells that will be of use to it later in its life. 



In most other animals this differentiation of somatic from repro- 

 ductive cells does not take place, as far as we can see, at so early a period, 

 although the latter can be traced back to a fairly early stage in many 

 forms. Figure 380 shows a bit of reproductive tissue in a very young 

 embryo of the dogfish, Acanthias, in which three germ cells are for the 

 first time visible. In most other forms it is not until much later that the 

 reproductive cells can be distinguished, sometimes not until the body is 

 well outlined in structure. In some low forms most of the somatic cells 

 appear never to give up their power of reproduction and are differentiated 



