298 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 (13). 



(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. novse-zealandise 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 



