228 N. M. STEVENS, 
HAIN’s iron-haematoxylin method, and all the figures were made from 
such preparations. 
Ovogenesis. 
No careful study of the ovogenesis of Sagitta has, so far as I 
know, been published. ©. HERTwIG (1880) and Grassi (1883) give 
figures and general descriptions of the ovary, and discuss the function 
of the so-called sperm-oviduct, the probable point of egress of the 
eggs from the ovary into the oviduct, and the possibilities in regard 
to the time and place of the entrance of the spermatozoa; but neither 
of these authors gives any account of the nuclear changes that occur 
during the development of the ova. Boveri (1890) determined the 
number of chromosomes in the nearly ripe oocytes, described and 
figured the two maturation spindles, the pronuclei, and the first division 
spindle. 
My material includes embryos 16--18 hours from the time of 
laying, and adult animals with ovaries in various stages. 
The embryos show the two large germ cells preparing for the 
division which separates the male from the female reproductive cells. 
Here 18 chromosomes, the somatic number, are present, as very long 
bands in earlier stages, and contracted to an ovoid form at the time 
‘when the nuclear membrane ig dissolving and centrosomes and spindle 
fibres are visible. 
The ovary of the adult animal, seen in cross-section (Pl. 20, 
Figs. 1 and 2), is nearly circular in outline and consists of an outer 
covering of endothelium, connecting by a short mesentery with the 
endothelial lining of the coelom; an oviduct — or in all probability 
merely a spermduct — with large deeply-staining nuclei and indistinct 
cell boundaries; and surrounding the spermduct, a layer of epithelium 
which extends out as a fold on either side like the arms of a crescent, 
and from the median central region of which the ova develop. Within 
the arms of the crescent are oocytes of various sizes (Figs. 1 and 2). 
Fig. 1 shows a section of a much smaller ovary than Fig. 2, the 
magnification being about two and one half times as great as in 
Fig. 2. Sections of still younger ovaries show essentially the same 
conditions, with the side wings of epithelium much shorter, only very 
young oocytes, and in very young ovaries, no lumen in the spermduct. 
In Fig. 2 the largest oocyte is nearly ripe, probably within a few 
hours of laying. The nucleus was drawn from another section of the 
same egg. 
