272 CLA YPOLE. [Vor. XIV. 
the central nucleus in centrolecithal eggs pass to the surface 
to form the blastoderm. Some may remain behind and form 
the vitellophags and perhaps take other part in development. 
It is certain in Anurida that vitellophags do not migrate out- 
wards and then return, but are left in the yolk. 
Development of the Reproductive Organs. 
The development of the germ cells was found to be one of 
the most interesting processes followed in detail. Their appear- 
ance takes place at a comparatively late period of embryonic 
life, the earliest stages occurring when the animal has reached 
the stage shown in Fig. 44.1 At this period the processes rep- 
resented in Figs. 50 and 51 are seen to take place in the second 
and third abdominal segments. Both these views represent 
cross-sections of the mesoblastic somites of one side of these 
abdominal segments, and show the great variation that occurs 
in the distinctness with which the cavities of the somites are 
developed. In Fig. 50 two germ cells (g.c.) are seen, one pass- 
ing out into the yolk on the free side of the somite, and one as 
yet imbedded in the splanchnic layer of the mesoderm. These 
cells are readily recognized by their peculiarly clear transpar- 
ent cell bodies. In Fig. 51 is shown another stage, when the 
germ cell is clearly inclosed in the cavity of the somite. Figs. 
2, 53, and 58 show the line of development followed in this 
latter case. The germ cells are distinctly between the walls of 
the two mesoblastic layers, the splanchnic and the somatic. In 
Fig. 53 a definite form has already been attained by the germi- 
nal mass. There is a cephalic elongation and a hinder spherical 
mass. The surrounding mesoderm has been differentiated into 
muscles, and connective tissue is beginning to appear. There 
is, however, a distinct layer of mesoderm separating the germ 
cells from the yolk, the splanchnic layer; the germinal mass 
lying in a space appearing to be a true body cavity. In Fig. 
58 the mass of cells is much larger, and by the crowding 
together of the abdominal segments it can be seen that flexure 
1 Since the reproductive organs are paired and the process is similar in general 
principles, descriptions will be made of but one side. The only difference lies in 
a slight variation in position of the two organs. 
