DEVELOPMENT IN PATIRIA MINIATA. 5 



I ) . It will be noted that the so-called " apical plate " is shown in 

 various forms, ranging from only a slight thickening to a small 

 secondary invagination or even several invaginations. Figure 9 

 shows one larva with two small secondary archentera. The physi- 

 ological interpretation of this process of twinning has been given 

 in another paper and does not especially concern us here. Suffi- 

 cient to say that the so-called " apical plate " is merely a secondary 

 region of low metabolic activity or a secondary area of primitive 

 endoderm, as is evidenced by the presence of yolk granules, and 

 that such a region very frequently invaginates and forms an addi- 

 tional archenteron. 



In the light of Heath's descriptions the above sounds like a fairly 

 dogmatic statement and one that should not be made without com- 

 plete proof. The proof of the above contention is to be found in 

 a study of the truly normal development of the species. If one 

 artificially fertilizes a good ripe lot of Patina eggs, having only a 

 single layer of eggs on the bottom of the dish, uses just enough 

 and not too much sperm, washes out excess sperm and any frag- 

 ments of ovary or testes after an hour or so, the following condi- 

 tions will be noticed within about twenty-four hours. A large 

 percentage of gastrulse like those shown in the camera drawings 

 (Figs. 13 and 14) are found swimming near the surface. These 

 are the normal larvae and such surface larvae rarely show any trace 

 of an apical thickening. Near the bottom of the vessel, however, 

 one finds numerous larvae of the types shown in Figs. 4-9, as well 

 as much more inhibited types. The normal larv,ae swimming near 

 the surface are perfectly typical asteroid gastrulae and are in no 

 way aberrant. If these larvae are skimmed from the surface and 

 placed in water taken fresh from the open sea, they develop as 

 shown in Figs. 15-23, showing no differences of consequence from 

 larvae of other asteroids studied side by side with them. 



Returning once more to Heath's account, we note that he claims 

 that " in fully 50 per cent, of the specimens in hand a few of the 

 mesenchyme cells arising from the blind end of the archenteron, 

 between the enterocele pouches, unite to form a small vesicle (Fig. 

 2, a, v). This usually occurs after the enterocele pouches are well 

 differentiated, though not completely cut off." Heath seems to 

 have seen such a thin-walled vesicle quite clearly, if one may judge 



