CLEAVAGE AND DIFFERENTIATION lOI 



A Special case is found in the Nematoda (Ascaris). Here the first 

 cleavage division is latitudinal, at right angles to the main axis, and 

 separates animal and vegetative portions. The developmental 

 potencies of the blastomeres have been tested by killing unwanted 

 ones with ultra-violet light. It is then found that the surviving 

 blastomeres develop just as they would have done under normal 

 conditions,^ and produce anterior or posterior half-embryos. How- 

 ever, by means of the centrifuge, the first cleavage division in these 

 eggs may be made to pass meridionally, and then both of the first 

 two blastomeres will develop a set of reproductive organs, i.e. will 

 produce more than they would normally have produced.^ The 

 regulative capacity of the Nematode egg before cleavage is shown 

 in the fact that fusion may occur between two eggs, which can then 

 regulate to form a single giant embryo of normal proportions.^ The 

 inclusion of the Nematode egg among "mosaic-eggs" is therefore 

 merely a consequence of the fact that in this group the first cleavage 

 division is latitudinal (see p. 398, and figs. 192, 193). 



The Echinoderms are of further interest in this respect. In some 

 forms, such as the star-fish Patiria and the sea-urchin Lytechinus, 

 at the earliest stage the apico-basal differentiation is absent, or at 

 least ineffective (see p. 313); both animal and vegetative halves of 

 unfertilised eggs, subsequently fertilised, are capable of giving rise 

 to normal miniature larvae.'* This is, however, not the case in 

 another sea-urchin, Paracentrotiis lividiis, for here, the apical organ 

 and stomodaeum-forming potency is restricted to the animal half, 

 and the gut-forming potency to the vegetative half of the un- 

 fertilised tgg. The Qgg can be cut into two equatorially, and then 

 both halves fertilised. The animal half will give rise to a blastula 

 with long cilia, the ciliation covering an abnormally large area and 

 thus forming a very diffuse apical organ, but such larvae have no 

 gut and no mesenchyme. The vegetative half will produce a larva 

 with a normally tripartite gut, mesenchyme and skeletal spicules 

 (the latter without any regular arrangement or orientation), but 

 without stomodaeum, cilia, apical organ, or arms. The same is true 

 for 4/8 animal and vegetative fragments. Animal and vegetative 



^ Stevens, 1909. - Boveri and Hogue, 1909, 



^ Zur Strassen, 1898. 



* Taylor and Tennent, 1924; Taylor and Whitaker, 1926. 



