SECT. 3] AND ORGANISATION 569 



contains some of the grey crescent region, which in time will develop 

 into the dorsal lip of the blastopore (Brachet) . With the ctenophora 

 we pass to a purely mosaic egg, for, as Fischel has shown, though 

 the normal ctenophore has eight combs, the result of a 2-cell stage 

 blastomere is one with four combs, that of a 4-cell stage blastomere 

 is one with two combs and that of an 8-cell stage blastomere is one 

 with only one. Obviously, then, something was distributed more or 

 less evenly among the blastomeres as the cleavages took place, and 

 cleavage of the cells involved equal cleavage of the comb-forming 

 substance. Crampton has discovered very similar phenomena in 

 molluscan eggs {Ilyanassa), Wilson in those of Dentalium, Driesch 

 in those oi Myzostoma, Stevens in nematodes, and Conklin in ascidians. 

 Loss of totipotence may even take place gradually before fertilisation. 

 The organ-forming materials may be actually visible in the un- 

 developed egg, as, for example, the brownish yellow pigment in 

 Styela eggs (an ascidian) which was found by Conklin to be necessary 

 for the formation of muscle. On the other hand, in some cases, as 

 Morgan has shown, pigments and formed elements are present 

 which, while normally distributed to certain cells, can be centrifuged 

 into a different position in the egg without in the least affecting 

 development. Other examples of this have already been given. In 

 1926 Duesberg found that some of the elements which seemed to be 

 necessary for development, and could not be centrifugally displaced 

 without damage, were mitochondria. But the most significant experi- 

 ments were those of Jenkinson, who found that normal morpho- 

 logical differentiation could go on in the frog embryo even after the 

 egg had been centrifuged and the fat globules had been made to 

 get into the "wrong" cells. In this case the brain was formed quite 

 normally, though it contained, at any rate as far as histological 

 examination could show, a much greater quantity of fat than usual. 

 Although this process could not be carried beyond a certain point, 

 it did show that fat alone was a structural material rather than 

 a specific organ-forming substance. These regulatory processes oc- 

 curring after centrifugation were shown by Konopacka for the frog's 

 egg to be remarkably independent of temperature. Totipotence phe- 

 nomena probably underlie the remarkable cases of Polyembryony 

 which occur sporadically throughout the animal kingdom (cyclo- 

 stomatous Bryozoa, earthworms, parasitic Hymenoptera, and Arma- 

 dillos) and which have been reviewed recently by Patterson. The 



