260 CLA POLE, [VoL. XIV. 
In living specimens the space between the embryo and the 
membrane is not very large, but it is practically constant; in 
the early stages the crenations are narrow and deep (Fig. 38, 
c.1), but later they are wider and shallower (Fig. 39, ¢.2). This 
is clearly associated with an increase in the size of the embryo, 
as measurements prove. Very soon after the formation of the 
membranes the first, the egg membrane, splits, and with it the 
vitelline membrane. Then the first crenated membrane becomes 
the outside cover, and a decided increase in size is observable. 
Growth continues until eventually the wrinkles are expanded 
so as to make the crenations flat in comparison with earlier 
stages. The attachment of this inner crenated membrane with 
the dorsal organ serves as a means of suspension of the embryo 
in the envelopes; it is thus held in a fixed position. In pre- 
served specimens, in which some amount of shrinkage of the 
embryo has taken place, the space round the embryo is consid- 
erable. The latter hangs eccentrically placed, owing to its 
attachment to the membrane. There is one other possible 
use for the crenations in the envelope besides the simple one 
of allowance for growth. The eggs are subject to considerable 
variations in pressure and degrees of moisture, owing to the 
changes in level of the tide. The crenated surface would 
more readily resist the effects of this change in pressure than 
an unfolded one. Observations were attempted to determine 
this point, but nothing definite resulted, and any such sugges- 
tion must remain an inference. 
It seems clear that powers of expansion belong to the embry- 
onic envelopes of at least three of the poduran genera, —Smyn- 
thurus, Anurophorus, and Anurida. As regards the causes of 
such changes, there is less known; in Anurida, growth and 
possibly changes of pressure are the direct agents, while in 
Anurophorus and Smynthurus contraction and expansion take 
place regularly without any apparent external or internal cause. 
Lemoine’s Fig. 16 strongly suggests another interpretation of 
the crenations found in the embryo; it so much resembles the 
early crenated stages of Anurida as to make it possibly a 
corresponding stage, instead of an embryo undergoing excessive 
contraction. 
