FERTILISATION 55 



usual manner, yet this gives the worst results in later development, be- 

 cause the eggs are left without a proper spindle mechanism to control 

 the cleavages. 



In other types of parthenogenesis new cleavage spindles may arise in- 

 dependently of the polar body spindles. For instance, the most effective 

 treatment for the parthenogenesis of frogs' eggs consists of pricking the 

 egg with a sharp needle. But this is effective only if the needle carries into 

 the egg some foreign protein. Normally sufficient such material is present 

 in the form of the cellular debris adliering to egg jelly. It was origin- 

 ally thought, e.g. by Bataillon, that it was necessary to inject a com- 

 plete nucleated cell. Recent studies by Shaver (1953), however, have 

 shown that the effect is actually brought about by ribo-nucleo-protein 

 granules. He made the interesting observation that the granules of this 

 kind present in the unfertilised egg are without effect, but they rapidly 

 acquire effectiveness just at the time when the blastula is developing 

 into the gastrula. This is interpreted by Brachet (1952^) as another mdica- 

 tion that the synthesis of new proteins begins to occur in the embryo at 

 that time. The action of these foreign proteins on the egg cytoplasm has 

 not been followed in detail, but there is no doubt that their main effect is 

 to cause an 'aster' or cleavage spindle to arise, possibly by some action 

 rather like that of coagulation. 



In some cases cleavage spindles appear to arise quite spontaneously 

 without any connection with polar body spindles or with introduced 

 foreign proteins. For instance, the eggs of some echinoderms can be 

 broken by strong centrifugation into a number of fragments of different 

 specific gravity. Only one type of fragment contains a nucleus; the others 

 are completely non-nucleated. They can, however, respond to activation 

 treatments similar to those which are effective on normal eggs. Asters 

 then appear in the cytoplasm and the fragments become divided up into 

 smaller lumps of cytoplasm which can apparently be regarded as cells, 

 except that they do not contain any nucleus (Harvey 1936, 1940&). This 

 absence of the nucleus is presumably responsible for the fact that the cleav- 

 age figures remain as single asters and do not unite in pairs to form spin- 

 dles. Nevertheless the 'cleavages' to which they give rise continue for a 

 considerable time and occur with some regularity. It seems that the 

 centrosomes around which the asters are organised are fully normal, and 

 are therefore endowed with the property of genetic continuity. As Tyler 

 (1941) has pointed out, this means that bodies endowed with the capacities 

 for identical self-duphcation and division can arise spontaneously in the 

 cytoplasm. It might be said, indeed, that we have here an instance of the 

 appearance of new plasmagenes (cf. Chapter XVIII). 



