CLEAVAGE AND DIFFERENTIATION 103 



is capable of being isolated, at the 32- or 64-cell stage, and studied 

 in respect of its developmental potency ^ (fig. 47). 



An isolated an. i disc develops into a blastula covered all over 

 with the long stiff cilia characteristic of the apical organ. An ap- 

 parent regulation later occurs in that these sensory cilia are lost and 

 replaced by mobile short cilia, with which the larva swims about. 



An isolated an. 2 disc develops into a blastula, three-quarters of 

 the surface of which are at first covered with the large stiff cilia. In 

 both of these two cases, a true pluteus larva is never formed. 



An isolated veg. i disc develops into a larva which may or may 

 not possess an apical organ. Ordinary cilia are present, and a small 

 gut is invaginated. 



An isolated veg. 2 disc produces a larva without an apical organ 

 but with cilia, and a gut is invaginated which may become tripartite 

 in the normal manner. 



The micromere group when isolated produces a ball of cells 

 which soon falls apart. Disc veg. 2 together with the micromeres 

 produces a larva in which the gut is so disproportionately large that 

 it fails entirely to invaginate: instead it protrudes outwards and 

 forms a so-called exogastrula. 



It is clear, therefore, that not only are the potencies of the animal 

 half different from those of the vegetative half, but that these 

 differences are graded along the main axis of the ^^g. 



As a result of this differentiation, whereas two sea-urchin eggs or 

 blastulae can give rise to a single double-sized larva when united 

 with their primary axes parallel, union with divergent axes results 

 in a double monster. The same applies to the results of uniting 

 two previously separated 1/2 blastomeres- (fig. 48). 



The Echinoderms present another curious phenomenon. Iso- 

 lated 1/2 or 1/4 blastomeres, though they give rise to whole larvae, 

 cleave as parts (see p. 128); e.g. a 1/2 blastomere will form four 

 mesomeres, two macromeres, and two micromeres, just as it would 

 have done if it had been left forming part of a whole ^gg : the early 

 blastula too is clearly a half and not a whole. If the consistency of 

 the cytoplasm in the developing Echinoderm tgg were so stiff as to 

 prevent a half or quarter blastula, produced in this way, from 

 rounding up into a sphere, the fragment could not have formed a 



^ Horstadius, 1931. - References in von Ubisch, 1925. 



