The early development of Eudendrium. 271 



hydroid. The extended investigations of this last feature by Weis- 

 mann (1883), JicKELi (1883) and others render such unnecessary. 



Interstitial cells. These cells make their appearance about 

 the time of the distinct differentiation of the true ectoderm as very 

 minute nuclei usually located among the basal, or interior ends of the 

 elongated and larger ectoderm cells. They continue small, and of a 

 more or less oval or pyriform shape, with a tendency to form clusters 

 or nests in a given place. Their later history following the liberation 

 of the planula I have not undertaken to follow. 



Entoderm Formation. 



From the preceding account of the origin and development of 

 the ectoderm it will be at once apparent that the formation of the 

 entoderm must be somewhat diöerent from the process usually in- 

 volved in this phase of development. Whether in Coelenterates or 

 animals of more complex organization. Certain it is that neither in- 

 vagination, delamination nor multipolar immigration, is involved 

 in the development of Eudendrium. With the proliferation of cells 

 centrally and the migration and further proliferation of certain of 

 these to form the ectoderm, and with the continued proliferation of 

 others till a central undifferentiated mass of nuclei forming a syn- 

 cytium, occurs nothing comparable with the phenomena involved in 

 the above named processes can be recognized. Furthermore, it is 

 quite evident that up to the time of the escape of the larva no 

 definite .entoderm is yet formed at all (cf. Figs. 26—28). This con- 

 dition continues for some time during the free-swimming life of 

 the larva and it is only with the approach toward metamorphism 

 into the hydranth, that a final differentiation of a definite entoderm 

 takes place, involving only a portion of the internal, syncytial mass, 

 while remnants of yolk, nuclei and cytoplasm are gradually disinte- 

 grated, digested and serve as food for the young organism during 

 its final larval life. 



I have shown that a similar condition obtains in the development 

 of Pennaria (1900). Allen (1900) in describing the development of 

 Tubullaria crocea^ while giving no details as to entoderm development, 

 refers it to Hickson's type of "precocious delamination". A careful 

 review of her figures and a later study of her preparations convinces 

 me that it is extremely doubtful whether any delamination in the 

 true sense occurs here. Indeed I think the same is also true conerning 

 Brauer's account of T. mesembryanthemum (1891). 



