EMBRYOLOGY. 249 



tions with those pertaining to the development of any other class of animals, 

 but the difficulty, it appears to me, arises at the outset when the cells included 

 within the test are declared to be wholly endodermic. I shall endeavor to 

 show that they comprise the elements of all three germ layers, and that the 

 Solenogastre development may be derived from that obtaining in the Chitons 

 for example. 



In the first place I am in entire accord with those authors, notably Drew 

 (1901), who look upon the test as a highly developed velum. In Ischnochiton 

 it is a relatively insignificant structure forming an equatorial band around 

 the embryo and dividing the animal hemisphere from the vegetative, or, roughly 

 speaking, the head from the trunk. Until the metamorphosis it remains 

 functional when it is cast off. RemodelUng such a type of larva into that of 

 Halomenia it is necessary merely to greatly widen the band. This well nigh 

 obliterates the usual head vesicle, leaving only those cells at the animal pole 

 that develop the cerebral ganglia. In the vegetative half of the animal the cells 

 responsible for the development of the trunk ectoderm are likewise greatly 

 reduced though by no means wiped out entirely. In both types the blastopore 

 is situated on the ventral side adjacent to the velum or test, and the ectoderm 

 that forms the future trunk is continuous with the margins of the test. 



The absolute proof of this theory rests upon a knowledge of the history 

 of the early blastomeres, and this unfortunately is almost totally lacking. We 

 know that the early cleavages may be nearly equal or highly unequal, but to 

 assume that because one or more cells become partially withdrawn into the 

 interior at an early stage it is therefore endodermal is certainly not justified. 

 Furthermore it is unfortunate that the terms micromere and macromere have 

 been introduced in describing the Solenogastre development since these terms 

 apply to the history of the cells in question rather than to its size. We know 

 that in the majority of the Trochozoa that have been carefully examined, the 

 original four cells divide three times forming thi-ee quartettes of micromeres, 

 some of which as a matter of fact may be larger than the remaining parent 

 cells or macromeres which are now endodermal. Whether this is true of the 

 Solenogastres we do not know, but the arrangement and fate of many of the 

 cells suggests strongly that something akin to this has taken place. 



Considering now the gastrulation of Myzomenia and Proneomenia it is 

 evident at the outset that the test is of such large sjze that when viewed from 

 the side it conceals within its borders the entire trunk. The so-called blasto- 

 pore is accordingly nothing more than a temporary shallow depression bordered 



