DIFFERENTIATION, HEREDITY, SEX 271 



consist of a mass of muscle and mesenchyme cells with a 

 double row of caudal endoderm cells, as in the corresponding 

 region of a normal larva. Equivalent results may be obtained 

 by injuring one or three cells of the four-cell stage (Fig. 125). 



The work of Roux, Fischel, Wilson, and many others has 

 demonstrated similar localizations in the eggs of many forms 

 the frog, other Ascidians, several Molluscs, Annulates, and the 

 Ctenophores, but we must limit ourselves to the mention of 

 only a few interesting details of the experiments on these 

 forms. 



The Mollusca afford several very striking illustrations of 

 the effects of the removal of parts of the egg or of blastomeres. 

 The egg of Dentalium, as described by Wilson, has an upper 

 clear area which normally forms the ectoderm, a middle reddish 

 or brownish pigmented zone forming endoderm, and a lower 

 clear area which during cleavage forms a peculiar "yolk lobe" 

 or "polar lobe" (Fig. 126). When this yolk lobe is entirely 

 removed from the segmenting egg the development of the 

 remainder proceeds as if it were present, and a larva is formed 

 which lacks the apical organ and the entire post-trochal region 

 (for explanation of terms see Fig. 126), and which develops 

 later into an organism lacking those structures which would 

 normally have been formed from this part of the egg and larva, 

 namely, the foot, mantle, shell glands and shell, pedal ganglion, 

 and apparently also ccelomic mesoblast. Other Mollusca give 

 essentially similar results although of course not all possess a 

 yolk lobe; but removal of blastomeres is always followed by 

 absence of specific parts in later development (e.g., Wilson, 

 Crampton, Conklin) (Fig. 126). 



The blastomeres of several species of animals fall apart, or 

 may be shaken apart easily, after a brief treatment with calcium- 

 free sea water, a fact discovered by Herbst and applied by him 



by the same letters, those referring to the right side being underscored. A and B 

 refer, respectively, to the anterior and posterior hemispheres. After the third 

 cleavage, all cells lying on the polar body side of that cleavage plane are desig- 

 nated by lower case letters, while those on the opposite side of that plane continue 

 to be designated by capitals. The first exponent following a letter indicates the* 

 generation to which the cell belongs. The second exponent refers to the position 

 of the cell relative to the vegetal pole. 



