226 LILTAN SHELDON. 



rounded with protoplasm, arrange themselves as a layer round 

 the embryo, in the formation of which they play no part. He 

 does not say that this layer is used as food material, but con- 

 siders it as a protective layer comparable in physiological 

 significance to the amnion of an ordinary insect development. 

 This process is very similar to that which occurs in P. novse- 

 zealandise, with the exception that the yolk is entirely wanting 

 in the eggs of Platygaster. The peripheral yolk layer probably 

 serves both as a nutritive and a protective layer, acting as a 

 shield for the embryo in its young stages, since it does not 

 become finally absorbed until the embryonic tissues have 

 acquired considerable consistency, and so would no longer 

 require such protection. Thus P. novse-zealandise seems 

 to have acquired by an extremely simple method an external 

 layer which serves at once the double purpose of nourishing 

 and protecting the embryo in its young stages. 



A somewhat similar result is also brought about, although 

 the means by which it is effected are quite different, in those 

 insects which undergo an internal development, and in which 

 the embryo is completely embedded in the yolk. The method 

 of effecting this is considerably simpler in P. novae-zeal a ndiae 

 than in these insects, nothing corresponding to the amnion 

 being present. It is possible that the amnion is a late develop- 

 ment, acquired for the protection of the embryo, and that on 

 its establishment it became involved with the external nutritive 

 mass. However this may be, it is clear that there are various 

 modes existing in Arthropods for the protection of the embryo 

 and the nutrition of the ectoderm, and that these differ very 

 largely in their mode of origin and in their structure, although 

 they resemble one another in their physiological functions. 



With regard to the small round bodies, which are so con- 

 spicuous a part of the peripheral layer, I have, as was said 

 before, no definite knowledge as to their origin. From their 

 form and structure one would be inclined to believe them to be 

 derived from the yolk, but this is militated against by the fact 

 that they stain a very deep red, whereas the yolk-spheres stain 

 bright yellow, and it is difficult to imagine that some of the 



