MUSEUM OF COMPARATIVE ZOOLOGY. 55 



and secondarily acquired connection with it, or that it had a double 

 origin. 



To Xitsclie ('71, pp. 456—463) belongs the credit of having first described 

 the histological changes in the origin and development of the polypide 

 of marine Bryozoa, particularly with reference to the part which the 

 germ layers play in that process. He says ('71, p. 456) : "Die Anlage 

 des Polypids erscheint zuuachst als eiue Wucherung der Zellschicht der 

 Endocyste in der Mitte der Hinterwand der Knospe, und zwar in dem 

 "VVinkel, den die Hinterwand mit der oberen "Wand macbt. Bald ordneu 

 sich die Bestandtheile des regelloseu Zellhaufens in zwei deutlich geson- 

 derte Schichten, und wir sehen nun eiuen rundlichen Korper, beste- 

 hend aus einer ausseren einschichtigen Zellschicht, welche sich scharf 

 absetzt gegen die das Innere des Korpers bildendeu Zellen." 



This stood until a year ago as the most satisfactory description of this 

 process in the adult stock. The appearance within the last year of the 

 two papers of Prouho ('90) and Seeliger ('90) marks a distinct epoch in 

 the advance of our knowledge concerning the origin of the polj'pide in 

 Gymnolsemata. The paper of Prouho treats of the process in the case of 

 the primary polypide of the metamorphosing larva of Fhistrella, tliat of 

 Seeliger in the case of the young (pi-actically adult) stock of Bugula, 

 According to both authors, the polypide arises from the body wall by an 

 invagination of it, and its two layers are from the first distinct and 

 separate, and go to form the two layers of the adult polypide, and the 

 whole of those two layers. The outer layer of the body wall gives rise 

 to the outer layer of the tentacles and the lining of the alimentary tract, 

 and the inner layer of the body wall gives rise to the mesodermal lining 

 of the polypide. Prouho alone is cognizant of the method of origin of 

 the ganglion, and in addition there are several points of difference be- 

 tween these two authors concerning the development of other organs, to 

 wliicli T shall refer in the proper place. Thus the latest studies have 

 confirmed the assertions of Xitsche, that the polypide arises from a 

 single centre of proliferation of the body wall ; they have made an ad- 

 vance in this, that they have shown that the two layers of tlie bud do 

 not become secondarily differentiated from a single cell mass, but are 

 respectively derived from the two cell layers of the body wall. My own 

 studies have led me to the same conclusion on this point. 



Figure 75 (Plate IX.) is a vertical radial section through the margin 

 of an adult Flustrella stock. The ectoderm is relatively thick at the sole 

 (sol.) and margin, and very greatly thickened at the point marked gm. 

 Here two layers, sharply separated, are apparent. The cells of the outer 



