CLEAVAGE AND DIFFERENTIATION 



97 



In the Hydrozoan Clytia, for example, if the blastomeres are iso- 

 lated at the 4-cell stage, all four of them can give rise to complete 

 little planula larvae which then settle down and develop into 

 hydroid polyps.^ To a certain extent, this totipotence of Hydroids 

 continues up to the i6-cell stage, at which isolated blastomeres can 

 still produce larvae, though apparently not polyps: whether this 



Fig. 44 

 Sea-urchin gastrulae and plutei from a whole egg (left) and a 1/2 blastomere 

 (right). The latter are normal except in size. (From T. H. Morgan, Sci. Monthly, 

 XVIII, 1924, p. 532.) 



is due to lack of material or to a real restriction of potency is 

 obscure. In the Ascidian Styela, on the other hand, even the first 

 two blastomeres, if isolated, will produce only half-embryos.^ It 

 is true that the ectoderm grows over the whole surface of the half- 

 embryo, that its notochord develops. to form a normally shaped but 

 half-sized rod, and that there is some rounding off of the general 

 form. But in its essentials, the organisation is that of a left or right 



1 Zoja, 1895, 1896; Maas, 

 ~ Conklin, 1905, 1906. 



^905. 



HEE 



