214 Edwin G. Conklin. 



is very different in cases of spiral and of bilateral cleavage, and 

 consequently the results of killing any one or more of the first four 

 blastomeres may vary in different cases; in general there is less 

 likelihood of obtaining an entire embryo from an isolated blasto- 

 mere of spiral cleavage than from one of the first two blastomeres 

 in bilateral cleavage. 



In other cases the cleavage planes bear no constant relation to 

 the planes of localization. Thus in the frog's egg the first cleavage 

 may lie in the median plane or at varying angles to this plane and 

 Brachet ('04) has recently shown that the character of an embryo 

 derived from one of the first two blastomeres depends entirely upon 

 the relation between the first cleavage plane and the median plane 

 of organization. 



It is probable that the bilaterality of organization is no more 

 perfect in ascidians than in annelids, mollusks or amphibians, but 

 the bilaterality of cleavage is much more perfect. Accordingly, 

 each of the first two blastomeres of the ascidian egg always con- 

 tains half of every ooplasmic substance, in the frog's egg it may 

 or may not contain half of these substances, in the annelid or 

 mollusk it never does. 



I agree therefore with Brachet ('04) and Wilson ('04S '04^) 

 that the varying results of experiments on the potencies of blasto- 

 meres are due in part to the varying relations of cleavage to local- 

 ization, and in part also to the different types of localization 

 (concentric, radial, bilateral) in different eggs. 



4. Determinate and Indeterminate Cleavage and Development. 



In a great many animals belonging to phyla as widely separate 

 as Ctenophora, Polyclada, Nemertinea, Nematoda, Rotifera, 

 Annelida, Mollusca, Arthropoda and Tunicata the cleavage of the 

 egg is constant in form and differential in character and under 

 normal conditions, definite cleavage cells always give rise to defin- 

 ite structures of the embryo or larva. For this type of cleavage 

 I proposed several years ago ('97, '98) the designation "deter- 

 minate." In a few animals the cleavage is known to be extremely 

 irregular, as in Pennaria (Hargitt, '04), Renilla (Wilson, '84), and 

 probably also in planarians (Hallez, '87; Stevens, '04), while in 

 other cases it is unknown whether the cleavage is normally con- 

 stant and differential or not (Echinoderms); in still other cases 



