The American Naturalist. [j nuary, 



;, namely the four encephaloblasts, two somatoblasts and six 



These four micromeres form the digestive tract, the other cells the 

 ecto- and mesoblasts. The four yolk masses at first have but one 

 nucleus each, but subsequently by what seems amitotic division, new 

 nuclei, accompanied by a little protoplasm, arise in the yolk and 

 envelops it in such a way as to preserve the original lines of cleav- 

 age between the four masses. These come to lie around, not in, the 

 digestive tract, the yolk being still in four masses in the latest 

 embryo. 



The persistence of these cleavage planes of the micromeres enables 

 one to refer them to the subsequent planes of the animal. 



The animal pole is anterior, and the first two meridianal planes 

 result in the formation of two smaller central cells and a small and a 

 larger dorsal cell, the larger cell being on the left of the animal. 



Without entering into the author's comparison between the phe- 

 nomena found in this Nereis and those recently discovered in other 

 Polychsetse, Oligochsetse and leeches we must refer to the way in which 

 the two somatoblasts give rise to the trunk as it bears upon the ques- 

 tion of " teloblasts." The first formed somatoblast divides into four 

 cells in a transverse row, and these again divide to form a parallel 

 row from which other cells bud off, making twelve in four longitudinal 

 rows, two on each side the median plane. The division of the second 

 somatoblast results in the formation of three transverse series, of which 

 the upper is composed of five or six larger cells that seem to give off 

 the other cells in longitudinal rows. These six larger cells sink in 

 and as myoblasts form the musculature, while all the other descendants 

 of the somatoblasts form nervous system, seta3 sacs, etc. 



Obviously all these cells represent the "ventral plate" found by 

 Wilson in a Heteronereis, while the two somatoblasts are identical 

 with the " primary toloblasts," though Wistinghausen seems to avoid 

 the use of the term teloblast as long as there are no long lines of cells 

 leading directly from a mother cell to the formed organ. 



The prototroch forms but a rudimentary simple band of ciliated cells 

 rotating the embryo. The anus would appear to arise at the point of 

 closure of the blastopore. The formation of organs in the trunk, on 

 the surface of a spherical mass of yolk has much to recall stages in 

 the embyology of an arthropod. Finally the head and trunk organs 

 unite when the oesophageal commissures are formed by growths from 

 both foundations. 



