226 CLA YPOLE. [Vor. XIV. 
yolk secretion being performed by either one or the other. In 
the case of Anurida the inference is that the nutritive cells 
take an active part in the yolkformation. Their great growth 
during the early stages of the egg, the presence of differentiated 
material in them, its disappearance, and, following this, the 
consequent appearance of yolk in the egg, the continued vigor 
of these cells until the full size of the egg is attained, and then 
their gradual absorption, are all facts pointing to the functional 
activity of the nutritive cells and their direct relation to yolk 
formation. 
Fig. 11 shows the nutritive cells at their stage of greatest 
development. The increase in the size of the eggs and the 
consequent crowding in the body cavity have together given 
the nutritive cells angular instead of rounded outlines. It is, 
moreover, hard to determine always to which egg they belong. 
There is no increase in their number from the beginning. 
The next step is the formation of the egg membrane and 
the degeneration of the nutritive cells. From the very slight 
development of the follicular cells it might be inferred that 
they take no part in the formation of the envelope of the egg. 
This is found to be true, and the process of formation by the 
egg itself is as follows: When the full size is reached the outer 
surface of the yolk undergoes a process of disintegration. 
Large yolk granules are broken up into smaller ones, and 
these in turn are finally transformed into a thin protoplasmic 
layer which shows clearly its network structure. This mesh- 
work gradually becomes obliterated, first on the outside and 
later farther in, as it were by condensation, until it shows an 
almost homogeneous structure outside preserving its fibrillar 
character inside (Fig. 12, ¢.m.). From the beginning, it stains 
deeply with Lyon’s blue, a fact true of the envelope of the egg 
in later stages. Its formation is almost simultaneous over the 
whole surface, excepting in the region of the nutritive cells, 
where it appears a little later, a fact observed by Korschelt 
(89a) in other insects. Fig. 12, e.m., shows the envelope in 
an advanced state. The hardened homogeneous exterior already 
exists and the nutritive cells are separated from the ovum. 
They have shrunk in size; the cytoplasm has almost entirely 
