62 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
increase in size partly in consequence of the enlargement of a cen¬ 
tral vacuole in each. Other chromatic bodies, which in the younger 
ova occupy a central position in the nucleus, either migrate out to 
the nuclear membrane or, as is more probable, break up into small 
masses of chromatin, for in the full-grown ovum there are no 
central nucleoli, but there are a small number of highly vacuolated 
chromatic bodies in contact with the nuclear membrane. Sections 
of the ova of Cucumaria frondosa enable me to confirm in detail 
Jensen’s (’83) observations upon the vacuolated condition of the 
nucleoli of this species. 
Yolk nuclei, such as have been described by Van Bambeke (’83), 
Blochmann (’84 and ’86), Henneguy (’93), Jatta (’82), Schtitz 
(’82), Stuhlmann (’86 and ’87), and others, make their appearance 
in the cytoplasm during the growth of the ovum (Plate 7 , figs. 
95, 97). They are spheroidal, somewhat less deeply stainable than 
nucleoli, and each is usually enclosed within a clear space in the 
cytoplasm. 
During the period in which the ova are growing rapidly, especially 
during the months of October and November, a small number of 
degenerating eggs are found in nearly every series of sections through 
the ovaries. They occur in material which is in all respects well 
preserved. The cells which degenerate at this time are smaller and 
less mature than many of the ova in the tubules, some of which have 
a well-developed follicle and zona radiata, the degenerating ova 
having neither of these envelopes. 
They consist of rounded masses of yolk of various sizes (Plate 8 , 
fig. 102), within some of which traces of a nucleus are found. 
Although these degenerating cells are not abundant, they possibly 
contribute to some extent to the support of the growing ova. It is 
probable that the substance of the degenerating cells is absorbed 
directly into the blood plasm of the internal lacunae of the ovary. 
Testes and spermatogenesis. The sexual cells in the testes of 
Caudina are extremely unfavorable for study on account of their 
small size ; I have therefore given comparatively little attention to 
this part of the subject. 
In cross sections of the genital tubules of a young male (Plate 8 , 
figs. 106, 107) in which no fully developed spermatozoa are present, 
germ cells of three distinct sizes are present. The largest of these, 
the spermatogonia (sp^go.) are found lying against the inner surface 
of the wall. The average dimensions of these cells are about 8 /x X 
