NOw25] ANURIDA MARITIMA. 247 
centrosome to the same source. On the other hand, Stuhl- 
mann and others fail to determine the nuclear origin, their evi- 
dence pointing to the cytoplasmic nature of the centrosome. 
As far as Anurida is concerned, there is no clear evidence of a 
_ nuclear origin. The yolk nucleus always appears in the cyto- 
plasm close to the nucleus and on one side, but there is no 
reason to think that it originates more from one than from the 
other: it has more the appearance of being the result of the joint 
activities of both. At least, this question cannot be settled 
without determining the greater one, the origin of the centro- 
some, if such an homology as has been suggested be accepted. 
Unsegmented Ovum. 
The eggs of Anurida when freshly laid are easily recognized 
by their peculiar light yellow color and smooth, shining surface. 
They are found fastened together in masses, the number of eggs 
in a mass varying from a few, five or six, to at most fifteen or 
twenty; each egg is about 0.27 mm. in diameter. These masses 
are sometimes arranged irregularly, the eggs being simply fas- 
tened together in a pile, as shown in Fig. 15, @; or there may be 
a definite form to the mass, owing to the position of the eggs in 
two rows in which the eggs alternate with each other (Fig. 15, 0). 
Very frequently in the central part of an ovum can be recognized 
a lighter spot; this shows the position of the central mass of 
protoplasm in which the cleavage nucleus is placed. The egg 
membrane is closely adherent to the surface of the egg in these 
early stages, but before cleavage takes place a distinct space has 
appeared between the membrane and the egg. No further points 
of differentiation are visible from the outside, but the internal 
structure is marked. Pl. XXII, Fig. 30, showsa section through 
an ovum having the external characters described above. The 
outer egg envelope and the vitelline membrane, which is very 
thin but distinctly formed by this time, are both omitted in the 
figure. The egg itself is seen to be formed at this time of a large 
central mass of protoplasm (Fig. 30, ¢.g.) with many radiating 
strands (7.), which branch as they pass to the periphery, and 
in most cases finally connect with a thin protoplasmic layer 
