124 John H. Gerould 



are those of Metschnikoff (1869) and of Selenka (1875). The 

 former makes this one statement: "Bei Phascolosoma (dessen Ent- 

 wicklung ich vor Kurzem in Triest beobachtet habe), bildet sich am 

 Embryo sehr früh ein bauchliegender Keimstreif, welcher ganz die- 

 selbe Bedeutung wie bei Chaetopoden hat. Außerdem kommen an 

 ihm solche Wimperapparate zum Vorschein, welche auch bei den 

 meisten Chaetopodenlarven vorhanden sind." 



Selenka studied at Villefranche near Nice the development of 

 a Phascolosoma which he calls Ph. elongatum Kef., but which is 

 surely not identical with the Ph. elongatum which Kefekstein (1863) 

 originally described from specimens collected at St. Yaast. This is 

 the smaller of the two species which are found in great abundance 

 at RoscoflF. The eg^ of the form at Villefranche is described by 

 Selenka as spherical, surrounded by a perforated yolk membrane, 

 through the pores of which protoplasmic processes project; that of 

 Ph. elongatum of the English Channel I have found to be ovate and 

 without protoplasmic processes. I have observed, moreover, that the 

 young larva of the latter form lacks the paired lateral bristles and 

 the circlet of hooks, which Selenka found on the prostomium of the 

 southern variety. He observed the giving off of a polar body, or 

 "Protoplasmatröpfchen", which he interpreted as possibly an excre- 

 tion, and describes the appearance of the tgg in the four-cell stage, 

 when it closely resembles that of Ph. gouldii and Ph. vulgare. The 

 polar globules of the ovate ^gg of Ph. elongatum of Roscoff, how- 

 ever, appear, as I have several times observed, at the blunter pole, 

 and the first cleavage plane divides the ovum obliquely into two 

 cells of unequal size. 



Selenka was unable to follow the segmentation with any 

 clearness beyond the four-cell stage. All of the smaller cells produced 

 by the earlier part of the cleavage, which he believed to represent 

 essentially the ectoderm, overgrow and enclose a large cell (Reserve- 

 kugel), from which the entoderm and "Blutkörperchen" are said to 

 arise. The large cell, dividing, forms "freie Blutzellen" on the one 

 hand, and in part a solid invagination, the entoderm. A lumen, the 

 outer opening of which is believed to be the mouth of the embryo, 

 is formed in this infolding by the separation of the cells. The 

 endoderm on the second day fuses with the dorsal ectoderm, and 

 the proctodaeum is formed by a breaking through of the latter. 

 It is impossible to identify Selenka's "Reservekugel" with any 

 particular cell. The difficulties in following the cleavage are so 



