DIFFERENTIATION, HEREDITY, SEX 269 



is fully described by the same author and it is clear that each 

 of these substances becomes contained within the cells forming 

 the rudiment of the particular organ or tissue mentioned. 



There are few other instances known where it is quite so 

 easily possible to distinguish the specific formed substances in 

 the egg. But in many ova, before cleavage it is possible to 

 distinguish several kinds of substance; thus there are known 

 in the eggs of certain Echinoderms four differentiated materials 

 (Lyon), three in Hydatina (Whitney), Dentalium (Wilson), 

 Physa, Lymncea (Conklin), four in Cumingia (Morgan), three 

 in the frog (McClendon), etc. (Figs. 42, 45, 86, 91, 123, 126, 

 129). 



The term " organ forming" as applied to these materials does 

 not rest alone upon the observation of normal development, 

 but upon experimental grounds as well. For an example of this 

 kind of evidence we may return to the work of Conklin on the 

 egg of Cynthia. If one, say the right, of the blastomeres of 

 the two-cell stage is injured so as to prevent its further develop- 

 ment, the remaining blastomere develops as it would normally, 

 i.e. , into the left half of an embryo and larva, containing approxi- 

 mately one-half the number of cells found in the normal 

 organism at the corresponding stage (Fig. 124). Embryos 

 derived from the two anterior cells of the four-cell stage never 

 possess a tail or any muscle cells, while chorda cells and 

 neural plate cells differentiate normally, and the endoderm and 

 covering ectoderm are also typically formed (Fig. 125). Corre- 

 spondingly, embryos derived from the two posterior cells have 

 no chorda, nerve, or sensory cells, or gastral endoderm, but 



endoplasm has also passed to the vegetal pole. The germinal vesicle has broken 

 down, and the maturation spindle is in the process of formation, between the 

 two asters. The residual substance of the germinal vesicle is clearly seen. 



C. Axial section through secondary oocyte, thirty-two minutes after fertilization. 



D. Longitudinal section, first cleavage; late anaphase. Posterior end toward the 

 left, anterior toward right. The ectoplasm of the polar lobe has been separated 

 from the remainder. E. Sagittal section through stage of about sixty-four cells. 

 The small upper cells are the apical cells. The ectoplasmic defect will be noted 

 in the posterior apical cell to the observer's right. A, animal pole; c, chromatin; 

 c.v., chromosomal vesicles of daughter nuclei; E, ectoplasm; e.a., e.b., e.c., endo- 

 plasm a, 6, and c. E.d., ectoplasmic defect; En, endoderm cells; m, mesoblast 

 cell; n, nucleolus; p.L, polar lobe; r.s., residual substance of germinal vesicle; 

 s.a., sperm aster; s.n., sperm nucleus; V, vegetal pole; X, derivatives of first 

 somatoblast. 



