The Animal Organisin and its Germ-Layers 51 



separated from the parent is exceedingly simple, being an 

 almost perfect sphere. The layers are, at this stage, only 

 a single cell thick and arc quite uniform throughout. The 

 endodermal layer, or "inner vesicle" as it is spoken of 

 technically, is separated from the ectoderm or "outer ves- 

 icle" by a wide space all around. Because of these simple 

 conditions the investigator is able to make out with great 

 certainty most of the events in the transformation of the 

 vesiculate stage into the completed organism. The first 

 differentiating step noticeable in the inner vesicle consists 

 of a somewhat elongated outpocketing of the wall of the 

 dorsal side of the vesicle. What occurs later in connection 

 with this outpocketing may be stated by quoting from the 

 original paper: "Simultaneously with the closing off from 

 the inner vesicle from before backward of the hypophyseal 

 duct, the ganglion becomes differentiated in the same order 

 from the cell mass that forms the last connection between 

 the duct and the vesicle." The "ganglion" is, it should 

 be stated, the beginning of the whole central nervous system 

 of these animals. My observations being a confirmation and 

 extension of those by other zoologists on other species, not- 

 ably by the older zoologists Giard and Kowalevsky, and in 

 the period of recent methods, by Hjort, there can be no 

 question that the nervous system arises in some gcmmipar- 

 ously produced ascidians, from the inner germ-layer whereas 

 in individuals of the same species produced from eggs, the 

 nervous system arises as it does in the vast majority of 

 animals from the outer germ-layer. 



The only point that has been or can be made against 

 this as an instance of complete transfer of the place of 

 origin of the nervous system from one germ-layer to another, 

 is that the "iiuicr vesicle" of the bud is not in reality endo- 

 derm but ectoderm, this resulting from tlie manner of de- 

 velopment in the parent of the layer from which tlie inner 

 vesicle originates. There is considerable ground for this 



