PHYSIOLOGICAL CHARACTERISTICS OF AXIATE PATTERNS 143 



tion of a secondary acropetal gradient, and in spite of the hypothesis of 

 Ranzi and Falkenheim (p. 139) that there is no gradient, although their 

 data indicate one, it appears evident that a gradient pattern of some sort 

 is a fundamental factor in early echinoderm development. This will per- 

 haps become even more evident in the following chapter. It remains for 

 the future to determine to what extent use of different methods and pro- 

 cedures, of different species as material, and possibly preconceived views 

 may be concerned in the lack of agreement and lack of information on 

 certain points. In later chapters some analysis of other lines of recent ex- 

 periment on echinoderm development will be attempted. 



OTHER INVERTEBRATES 



A few data are at hand concerning mollusks. Early developmental 

 stages of several gasteropods show basipetal decrease of susceptibility, as 

 indicated by cytolysis and death and later a local increase in the region 

 which becomes the shell gland (Child, unpublished). Differential suscepti- 

 bility to various agents has been observed in early cephalopod develop- 

 ment (Ranzi, 1927; 1928; 1929a, b). In gasteropods and a cephalopod the 

 apical region of the egg becomes more alkaline than the basal (Spek, 

 1934a; Ries und Gersch, 1936). Methylene blue leucobase (rongalite 

 white) oxidizes most rapidly in the apical region of the egg of Aplysia, 

 an opisthobranch gasteropod. The more rapid oxidation of the leucobase 

 by single blastomeres, even by single or certain micromeres, observed by 

 these authors, raises certain questions. Oxidation is more rapid in one 

 cell of the two- and four-cell stages and in some of the micromeres of later 

 stages. Since the prototroch of the early larva also oxidizes leucobase 

 more rapidly than other cells, they regard the differential oxidation in 

 earlier stages as indicating cell lineage of the prototroch and velum from 

 one cell of the two-cell stage and a single quadrant of the four-cell stage 

 and from certain micromeres of this quadrant. The cleavage is not fol- 

 lowed in detail ; and in the light of earlier studies on cell lineage in mollusks 

 and annelids, such origin of the prototroch appears improbable. The pos- 

 sibility suggests itself that difference in condition associated with stage 

 of the division cycle may be responsible for some of the differences in 

 rate of leucobase oxidation in individual cells; such difference may also 

 determine a difference in susceptibility to the toxic rongalite white, and 

 the deep staining of certain blastomeres may result from injury. These 

 authors also find that reduction of Janus green decreases in rate from 

 the basal pole acropetally in maturation and cleavage stages of Aplysia. 



