574 PATTERNS AND PROBLEMS OF DEVELOPMENT 



an isolated AB-ccW, or the cell containing all chromosomes may develop 

 like a whole egg (Faure-Fremiet, 1913). It seems probable that these dif- 

 ferences in development result from alterations in the apicobasal gradient 

 pattern by the inhibiting conditions and the following recovery. Eggs 

 allowed to develop by exposure to air after some 3 months in CO2 give 

 forms ranging from normal to unorganized cell masses, but a third of them 

 show various degrees of what appears to be a differential inhibition, more 

 or less of the anterior region being present merely as an unorganized cell 

 mass and posterior regions normally developed (Painter, 19 15). 



Sometimes a considerable part of the cytoplasm, usually containing the 

 nucleus of the second polar body, constricts off from the apical region but 

 does not divide. The remaining part of the egg divides into a dorsal and 

 a ventral cell, and these change their position with respect to this Neben- 

 zelle in the same way as the two ventral cells of the normal four-cell stage. 

 The following cleavages may be normal or more or less altered by presence 

 of the Nehenzelle; but, with a single exception, normal animals have not 

 been seen to develop from these eggs (Kautsch, 191 2). 



Although an apicobasal gradient pattern is indicated in the fertilized 

 Ascaris egg, it does not necessarily follow that regional cytoplasmic dif- 

 ferences are merely quantitative, but conclusive evidence for regional 

 specificity at the beginning of development is lacking. The undivided egg 

 appears not to be a mosaic, and the change in position of cells at the four- 

 cell stage suggests physiological relations of some sort between the blasto- 

 meres. As regards cleavage pattern, regional characteristics are apparent- 

 ly determined early, but independent differentiation of isolated blasto- 

 meres does not proceed very far. If the ventral cells of the four-cell stage 

 do determine the anteroposterior axis, they perhaps induce some degree 

 of anteroposterior differential in the dorsal cells. It is evident, however, 

 that the cleavage pattern of Ascaris gives us little definite information con- 

 cerning developmental pattern. 



ENTOMOSTRACAN CLEAVAGE PATTERN 



The normal cleavage pattern of Cyclops is determinate, at least as re- 

 gards mesoderm, entoderm, and primitive germ cells. The germ path is 

 indicated by certain cytoplasmic granules with a definite cell lineage from 

 the first cleavage to the primitive germ cells. The plane of first cleavage is 

 said to be in the shortest diameter of eggs deformed by pressure in the egg 

 sac; and, according to Fuchs (19 14) and M. Jacobs (1925), the cleavage 

 axis has a definite and constant relation to developmental pattern and be- 



