192 



INTERNAL FACTORS 



IV. i 



develop a normally tripartite gut (Fig. 100). But this only occurs 

 in a small proportion of cases, smaller for the animal than for 

 the vegetative cells. Further, the former frequently give rise to 

 blastulae provided with a row of long cilia (Fig. 101), while the 

 latter are delicate, and many die. 



Boveri has pointed out that the ability of animal cells to 

 gastrulate depends, in Strongylocentrotus, on whether or not they 

 contain a portion of the pigment-zone, and this on the position of 

 the third furrow. Garbowski, however, denies that the pigment 



is itself an organ-forming 

 substance. For in the first 

 place there is a variety of 

 Strongylocentrotus lividus, 

 in which the pigment 

 remains diffuse and never 

 becomes concentrated to 

 form a band at all ; and 

 secondly, even though, 

 when present, it is usually 

 in the subequatorial posi- 

 tion described by Boveri, 

 this is not always so ; it 

 may be oblique to the 

 egg-axis, or wholly in the 

 animal, or wholly in the 

 vegetative hemisphere. 

 It would appear, there- 

 fore, either that it is merely associated with some other sub- 

 stance which has up to the present remained invisible, or that 

 in the cases described by Garbowski animal cells, if they were 

 pigmented, would gastrulate as readily as ordinary vegetative 

 blastomeres, in which case the manner in which the pre-deter- 

 mined material is cut up in segmentation would be a matter of 

 indifference. 



The differences in the behaviour of the ^ cells are still more 

 marked. The micromeres will only divide to form a heap of 

 about ten cells ; the mesomeres give rise to either long-ciliated 

 blastulae or imperfect gastrulae, with or without skeleton and 



FIG. 101. Long-ciliated blastula from 

 a animal cell of Echinus. (After 



Driesch, 1900.) 



