ISOMETRA VIVIPARA. 43 



in Tropiomelra. Seeliger states that this mass consists of degenerating cells 

 derived from the entoderm c(^lls and having the object of serving as nourish- 

 ment to the embryo until it is able to take its own food from the surroundings. 

 I doubt very much that this is the correct explanation of the phenomenon. 

 First, on purely logical grounds. The meaning of this multiplying of the 

 entoderm cells, in order that the cells thus orginating may serve as food, 

 is, in fact, this, that the embryo is feeding ujion itself. Then, regarding 

 Isometm vivipara, it will be remembered that the entoderm in the fully 

 formed larva is filled with a yolk-mass in which some nuclei are scattered.^^ 

 (Plate XVI, figures 6 and 9). It seems hardly possible that these few nuclei 

 should have divided at such an enormous rate as to produce the immense 

 number of grains that fill the entoderm, during the short time which must 

 be supposed to be required for the fixation and the transplacement of the 

 vestibulum, judging from the cases of Antedon and Tropiomelra. 



The true explanation may be found by comparing this feature of the 

 Crinoid development w'ith what obtains in the Echinoid larvse during the 

 metamorphosis, for here also the entoderm is filled with a mass of small cells, 

 as first described by MacBride,^ who regards this process as a kind of his- 

 tolysis and, like SeeHger, states that the small cells filling the lumen of the 

 entoderm are produced by the entoderm cells, which are said to multiply 

 with great rapidity. More recently L. v. Ubisch '» has made a very careful 

 investigation of this process, his result being that a histolysis of the entoderm 

 really takes place, as stated by MacBride; but the small cells filling the 

 lumen of the entoderm are not produced by the entoderm cells; they are 

 wandering cells, originating from the mesoderm, which migrate through the 

 entodermal epithelium into the lumen of the entoderm. It could not be 

 decided, whether these wandering cells replace the original entoderm cells 

 or whether only the nuclei of the entoderm cells are dissolved during the 

 histolysis, to be replaced in some way or other after the metamorphosis. 



Although no nuclear structure could be discerned in the grains filling 

 the entoderm of Isometra, I have little doubt that we have to do with the 

 same thing as in the Echinoids — the mass occupying the entoderm consisting 

 of wandering cells, derived from the mesoderm, which must be supposed 

 to take an active part in the histolysis of the entoderm. That they are not 

 derived from the entoderm cells is indicated also by the fact that they are 



■' That this cell-mass is, as I think, meant to serve as nourishment to the embryo, is something quite 

 different from what Sccliger maintains. It is here, probably, only a sort of dilutinc of the rich yolk material 

 contained in the entoderm cells, in order to make it more easily digested, or, perhaps, it is essentially a 

 question of space, the multiplying cells being pressed out into the lumen of the entoderm, and, it should be 

 added, it is doubtless only the yolk material, not the nuclei and the protoplasmatic content of the cells, that 

 is digested. 



^'E. W. MacBride. The development of Echinus esculentus, together with some points in the 

 development of E. viiliaris and E. aculus. Philos. Trans., ser. b, vol. 195, 1903, p. 309. 



'"L. V. Ubisch. Die Entwieklung von Slrnngylocentrotus lividus {Echinus viicrolubcrcidatus, Arbacia 

 lixula), Zeitschr. f. wiss. Zoologic, Bd. 106, 1913, p. 433. 



