190 R. S. Bergh on the Development of the 



So soon, however, as the definitive number* is attained, these 

 cells commence precisely the same process as the primitive 

 cells of the muscle-plates (the cells lying nearest to the median 

 line are the first to begin, and are followed by those occupying 

 a more lateral ])osition) : they produce smaller cells in frout 

 by budding (" Knospung "). In this manner there conse- 

 quently arises an ectodermal germinal streak, formed of seven- 

 teen or nineteen longitudinal rows of cells. The cells of this 

 germinal streak are also seen to be arranged very regularly 

 in transverse rows, and the cell-divisions take place precisely 

 in the same manner and with the same regularity as I recently 

 described in tlie case of Gammarus. A median row of cells 

 is always found, which is derived from a median ])rimitive 

 cell ; therefore the number of the primitive cells and of the 

 rows of cells is always an uneven one. 



This ectodermal primitive streak extends in front as far as 

 a line which connects the points of insertion of tlie right and 

 left mandible. In front of this line we find in all stages under 

 consideration a mosaic of ordinary polygonal ectoderm-cells, 

 which are not arranged in rows and are not derived from the 

 above-mentioned primitive cells. It seems to me that this 

 fact, that the ventral ectoderm is differentiated , so to speak, into 

 a naiqAial and (sit venia verbo) metnnauplial rudiment, is not 

 entirely devoid of interest. The Nauplius appendages grow 

 out from the anterior mosaic of cells ; but the whole of the 

 appendages which are situated behind the mandibles owe their 

 origin to the germinal streak which is derived from the primi- 

 tive cells. Behind the primitive cells there is formed at an 

 early period an embryonic (provisional) forked caudal fin 

 (" Schwanzflosse "), which is very distinct in the Nauplius 

 stage. The epidermis withdraws by degrees from the chitinous 

 covering of this caudal fin, and the definitive caudal fin is 

 formed considerably further forward from the material of the 

 germinal streak. It is probable that the only other structure 

 which arises from the cell-material lying behind the primitive 

 cells is the telson, though this is difficult to prove. 



The position of the endoderm-plate in different stages is 

 worthy of notice. Situated at first behind the ectodermal 

 primitive cells, it travels by degrees on the inside of these and 

 of the germinal streak very far forwards, until it enters the 

 region of the mandibles. The primitive cells of the muscle- 



* The numerical theorists of Prague are here furnished with interesting 

 subjects for study. A comparison of the conditions of Mysl^ and Gam- 

 marus with reference to the validity of their laws would certainly have a 

 brilliant result. 



