102 CLEAVAGE AND DIFFERENTIATION 



halves isolated at later (blastula) stages show the same develop- 

 mental potencies, the only difference being^ that a stomodaeum is 

 formed in such animal halves (see p. i66, and fig. 46). 



Already in the unfertilised egg of this sea-urchin {Paracentrotus), 

 therefore, the cytoplasm of an animal half, which represents only 

 presumptive epidermis and other epidermal structures, lacks the 

 potency to form an enteron, while that of a vegetative half is in- 

 capable of forming an apical organ or stomodaeum.^ There is there- 

 fore an important distribution of potencies along the primary egg- 



a b 



Fig. 46 



Partial larvae from fragments of sea-urchin eggs, a, Blastula with abnormally 

 extensive apical organ, derived from animal half of unfertilised egg, fertilised egg, 

 or young blastula. Note absence of gut, mesenchyme, stomodaeum. b. Ovoid 

 larva, without stomodaeum, apical organ, ciliated band or arms, derived from 

 vegetative half of unfertilised egg, fertilised egg, or blastula. Note presence of 

 spicules and tripartite gut. (From Horstadius, Acta Zool. ix, 1928.) 



axis, and it is because they include all the levels of this axis that 

 the blastomeres of the 2- and 4-cell stages of Paracentrotus and 

 meridional halves of gastrulae (see p. 81) are totipotent. This dis- 

 tribution of potencies along the egg-axis has been further analysed by 

 studying the developmental potencies of pieces smaller than halves. 

 At the 32-cell stage, the cells of the animal half of Paracentrotus 

 form two plates or discs of mesomeres, one above the other. They 

 may be designated as an. i and an. 2 (presumptive ectoderm). The 

 cells of the vegetative half (at the 64-cell stage) also form two discs 

 of macromeres, which may be referred to as veg. 1 (presumptive 

 ectoderm) and veg. 2 (presumptive endoderm). Lastly, at the ex- 

 treme vegetative pole of the egg, there are the micromeres (pre- 

 sumptive primary mesenchyme). Accordingly, the egg of Para- 

 centrotus can be divided latitudinally into five layers, each of which 



^ Horstadius, 1928. 



