220 C. O. WHITMAN. 



deciding tliis question. I am unable, however, to explain the 

 presence of free nuclei in the r)i:ieliis, on any other hypothesis 

 than that here is the real place of ci'U-formation. In accordance 

 with this hypothesis, the egg-cell arises from a nucleated proto- 

 plasm, as is the case in most Worms, especially the unsegmenled, 

 and in many Arthropods (Crustacea). The structure of the 

 central part of the egg-string bears a striking resemblance to 

 the rhachis of the Nematoids, and for this reason I have given 

 it that name. We have seen (1st) that the rhachis, at the time 

 the voung eggs begin to api)ear, contains yolk- granules tolerably 

 equally distributed; (2nd) that the median part of the rhaciiis, 

 a little later, becomes charged with these granules; (3rd) that, 

 while the axis distinguishes itself as the place of most energetic 

 formation of such granules, this function is by no means localised 

 here; (4th) that in the rhachis free nuclei are suspended, around 

 which (hypothetically) the protoplasm differentiates into the cell- 

 form, thus giving rise to the peripheral cellular ])art of the 

 string. According to the researches of Claparede (31), Leuckart 

 (107), and van Beneden (13), all these relations are repeated in 

 the Nematoids. All this, together with the important fact that 

 in Nematoids the entire egg is produced by a single organ in- 

 stead of two as in the Trematoda, points to a nearer relationship 

 between the Nematoidea and the llirudinea than exists between 

 the latter and the Trematoda. Opposed to this stands the less 

 im])ortant fact that hermaphrodism is the rule among Trematodes 

 and the exception among Nematoids. 



h. Growth of the Primitive Egg- cell to the Mature Egg. 



The peripheral part of the egg-string, as before stated, is a 

 compact cellular layer four to five cells deep (fig. 57). 



These cells are the priman/ egg-cells, of which only a com- 

 paratively i'tvn at any one time develop into eggs. The first step 

 in this development is signalised by the accumulation of yok- 

 granules around the nucleus (fig. 58, a). At this time the 

 nucleus and nucleolus of the young egg do not differ, to any 

 appreciable i-xtent, in size and general appearance from those of 

 the surrounding cells. \\\ little later stages [b, c, d) the granules 

 have increased, and lie, for the most part, in the periphery of the 

 egg. The germinal vesicle and germinal dot have increased in 

 size, but not in the same proportion as the body of the egg, ;is 

 a{)pears from the following measurements of a and d in fig. 5S : — 



a. E?!* =: '02 nun. (longest diani.) ; germ. ves. = '01 mm. ; germ. dot. 

 = •0(125 mm. 



d. E'j:g = "10 mm. (longest diam.) ; germ. ves. = "03 mm. ; germ. dot. 

 = -005 mm. 



While the diameter of the egg has increased five-fold, that of 



