CLEAVAGE AND DIFFERENTIATION. 



homogeneous cells; such divisions are extremely constant and 

 in many cases are visibly differential. Even in the case of the 

 echinoderm egg it has been shown that four micromeres are 

 constantly formed at one pole 

 of the egg, and in this respect, 

 at least, the cleavage here 

 is determinate, for although 

 Driesch has shown that a nor- 

 mal larva develops from a sea- 

 urchin egg from which the mi- 

 cromeres have been removed, 

 this no more indicates, as 

 Morgan l assumes, that these 

 micromeres are undifferenti- 

 ated and that the cleavage is, 

 therefore, indeterminate than 

 the fact that a hydra is able to 

 complete itself and form a nor- 

 mal hydra after its tentacles 

 have been removed indicates that these tentacles are un- 

 differentiated. 



The one most striking feature of determinate cleavage is the 

 constancy with which certain blastomeres give rise to certain 

 organs, the invariable segregation of an entire region, layer, or 

 organ into a single cell or particular group of cells. In all the 

 gasteropods mentioned above the ectoderm comes from three 

 quartettes of cells, each of which occupies relatively the same 

 position and gives rise to the same organs (Fig. 4). The 

 mesoderm comes from the posterior cell of the fourth quar- 

 tette. All the other cells are entodermal, and, although they 

 show certain variations in size and position in different genera, 

 owing to variations in the amount and distribution of yolk, 

 they are always constant for the same species. The four 

 apical cells give rise to an apical sense organ (see Figs. 3-10), 

 the trochoblasts and tip cells of the cross form the first velar 

 row, the anterior arm of the cross forms the anterior cell plate, 



FIG. 7. Crepidula, log-cell stage (ninety-two 

 ectoblast cells). Shading and heavy lines 

 as in preceding figures. The egg is repre- 

 sented as if all the ectoblast cells could be 

 seen from the apical pole, though actually 

 many of the peripheral cells lie far down on 

 the sides, or even on the ventral face of 

 the egg. 



1 Morgan, T. H., " A Study of a Variation in Cleavages," Arch.filr Entwick- 

 lungsmechanik, Bd. 2. Hft. I. 



