24 LILIAN SHELDON. 
that if they existed I should have seen no trace of them, since 
they were in every case very prominent objects in both the 
Cape species; and even if the bodies themselves had become 
artificially detached, the depression in which they lay would have 
been visible and also the stage of the nucleus preparatory to 
their being budded off from it. Besides, the numerous cases 
described by Stuhlmann (17) in which they were absent, and 
it is hardly possible that he could have missed them in so 
many cases, point to the conclusion that they are not universally 
present in fertilized Arthropod eggs. The two polar bodies 
in the Cape species are exactly similar to one another, 
a condition which might hardly be expected if their meanings 
were so different as Weismann (18) suggests. 
Their presence in the Cape species and the absence in the 
New Zealand one suggests that they are in some way dependent 
on the yolk, since this is the main difference between the eggs. 
Formation of the Yolk. 
The question as to the origin of the yolk has received con- 
siderable attention, and various observers have brought forward 
several theories as to its mode of formation. The principal of 
these are : 
(1) That the yolk arose in the protoplasm of the egg itself. 
This view was upheld by Professor Balfour (2). Stuhlmann 
(17) states that this occurs in the Arthropod ova which he 
investigated. 
(2) From the breaking up of the germinal vesicle, As far 
as I know this mode of origin has only been recently described 
by Will (20) and Scharff (18). 
(3) From the follicle cells (inner capsule) as described by 
Lankester (12) in Sepia, by Will (20) in Nepa, and by 
Beddard (3) in Lepidosiren. 
P. nove-zealandiz is interesting as affording an example 
of ova in which the yolk is formed in all these three ways, all 
three being very clearly seen in sections of the ova. 
The yolk also arises from a fourth source, viz. from yolk 
