76 THE MAMMALIAN EGG 



spindle by centrifugation resulted in the formation of giant polar 

 bodies, sometimes as large as the egg itself. Tyler (1932) found 

 that unfertilized Urechis eggs placed in hypotonic sea water for 

 an appropriate period underwent complete cleavage into two 

 blastomeres instead of emitting polar bodies, and subsequently these 

 eggs developed into embryos. Tyler was able to show that the first 

 cleavage division had been effected by the presumptive polar spindle 

 which had migrated to the centre of the egg ; this mechanism, by 

 maintaining diploidy in the embryo, had evidently made possible 

 the parthenogenetic development (see also Tyler, 1941). Observa- 

 tions indicate that, in mammalian eggs, cleavage by a presumptive 

 polar spindle can occur both spontaneously and in response to 

 experimental treatment. Spontaneous cleavage of the egg by a first 

 maturation spindle has been reported in the dog (Grosser, 1927) and 

 mouse (Pesonen, 1946a, b; Braden, 1957). Braden cites an un- 

 published observation by R. G. Edwards and himself on a mouse 

 egg, cleaved at the first meiosis, in which one 'blastomere' had been 

 penetrated by a spermatozoon so that there is certainly a possibility 

 that one or even both cells of such eggs can undergo fertilization 

 and proceed with development. This could give rise to mosaic or 

 gynandromorphic individuals. 



Cleavage of mouse eggs at the second meiosis was found by 

 Braden (1957) to be much more common than that at the first. The 

 incidence varied with the stock or strain: 0-9 per cent (in 910 eggs) 

 in A strain mice, 0-3 per cent (in 604 eggs) in V stock, 0*2 per cent 

 (in 1,335 eggs) in J stock, 0-9 per cent (in 456 eggs) in JS stock and 

 0-4 per cent (in 232 eggs) inJNS stock; no examples were found 

 among 1,073 e gg s of CBA strain mice, among 749 eggs of C57BL 

 strain or among 645 eggs of RIII strain. When the cleavage took 

 place in an egg that had been penetrated by a spermatozoon, one 

 of the cells contained a male and a female pronucleus, and usually 

 the sperm tail as well, while the other cell contained only a single 

 nucleus similar in size to the female pronucleus of the first cell ; 

 sometimes the sperm tail lay partly in one cell and partly in the 

 other. Two-celled eggs with two nuclei in one blastomere and one 

 in the other, which may well have arisen in this way, have also been 

 described by Van der Stricht (1923) in the bat (Fig. 64), Austin and 

 Braden (1953b) in the rat, Austin and Braden (1954c) and Edwards 

 (1957a, b, 1958b) in the mouse and Hancock (1961) in the pig. 



