BIGELOW : EARLY DEVELOPMENT OF LEPAS. 129 



closing the blastopore. Ectoblastic cells around the blastopore give rise 

 to mesenchyme (" secondary mesoblast "), which forms most of the 

 mesoblastic structures of the Nauplius. The mesoblast proper probably 

 originates from the entoblast, as does the primary mesoblast of Lepas. 

 It is evident that there is in Cyclops, .according to Urbanowicz, a condi- 

 tion closely resembling that of Lepas. 



In close agreement with Urbanowicz's account of Cyclops and ray 

 own of Lepas, is Pedaschenko's ('93) description of the formation of 

 the germ-layers of the parisitic copepod Lerusea. In this genus the 

 mesoblast and ectoblast are separated from the yolk-entoblast in the 

 first four divisions, as in Lepas. The four micromeres thus produced 

 subdivide and form the blastoderm, which grows over the entoblast. 

 At the margin of the growing blastoderm (blastopore) some cells (ap- 

 parently ectoblastic) divide parallel to the surface and form migrating 

 mesenchyme cells. These apparently correspond to the " secondary 

 mesoblast " of Lepas. On the ventral side four of the cells sink beneath 

 the ectoblast and constitute the primitive mesoblast cells. The lineage 

 of these cells has not been definitely traced, but from their position I 

 infer that they are probably the direct descendants of the fourth micro- 

 mere, in which case the primary mesoblast originates directly from the 

 entoblast, as in Lepas. 



Hacker's ('92, '97) studies of Cyclops led to results widely different 

 from those of Urbanowicz. According to Hacker, a cell lying in the 

 blastopore divides into a genital cell and a primitive mesoderm cell. 

 The cells surrounding the blastopore divide, giving rise to the primitive 

 endoderm cells ; this is in line with Grobbeu's account of Cetochilus, 

 to which reference will be made later, and opposed to Urbanowicz, who 

 found mesenchyme cells originating from cells bounding the blastopore. 



Grobben's ('81) views of the formation of the germ-layers in the 

 copepod Cetochilus do not agree with the account of Cyclops given by 

 Urbanowicz, and only in part is there agreement with Hackei-'s account 

 of Cyclops. His description of the thirty-two-cell stage of Cetochilus 

 forms the best starting-point for purposes of comparison. In this stage, 

 viewed from the vegetative pole, there is noticed a distinct bilateral 

 symmetry in arrangement of the cells. A *' central entoderm " cell and 

 one small " anterior entoderm " cell lie in the median plane. Four 

 cells placed symmetrically on either side of the " central entoderm " 

 cell will by the next division form "entoderm" and ectoderm. The 

 cell in the median line and posterior to the " central entoderm " cell 

 forms in later division four cells, of which the two nearer the " central 



