EMBRYOLOGY OF COCCIDS 101 



for aphids. With Will, I therefore conclude that, in these 

 families of insects, namely, Aphididae and Coccidae, and perhaps 

 others, there is a process of true gastrulation as in other higher 

 animals, and that the invagination pore of our insects represents 

 the blastopore of other animals, because it is here that the 

 three germ layers arise. 



^Mention should also be made that, in the case of coccids 

 studied, the mouth and the anus arise, respectively, at the 

 anterior and posterior ends of the invagination. Accordingly, I 

 agree with Will, Wheeler, Kowalevsky, and others that the 

 blastopore of insects is homologous to that of other animals, 

 e.g., of amphibia, and that it is much elongate, even to the 

 extent of forming a loop within the egg. 



As previously stated, the entodermal cells in Pseudococcus 

 macdanieli become localized at the posterior end of the germ 

 band only. My interpretation of this unique phenomenon is 

 this : On account of scarcity of yolk, the entodermal cells remain 

 in a more or less inactive state as compared with the rapidly 

 proliferating mesoderm cells beneath. Consequently, the former 

 become entirely cut off from the point of their origin, the blasto- 

 poric rim. If this interpretation be correct, it follows that the 

 scale insects are much more specialized than the Orthoptera, in 

 which, according to "V\'Tieeler ('93) and others, the entoderm is 

 found at the two extreme ends of the germ band. 



7. THE FORMATION OF THE NERVOUS SYSTEM 



The nervous system of the coccids studied is formed exactly 

 in the same manner as in the case of other insects, for example, 

 in Blatta germanica, so well described by WTieeler ('93). No 

 account, however, of neurogenesis in Hemoptera, — aphids, 

 coccids, and others — has ever been presented, so some details 

 of the history of the nervous system of the coccids may not be 

 out of place. 



Although the brain of the scale insects is but a continuation 

 of the ventral cord, and the ventral cord, in its genesis, is but a 

 part of the whole nervous system, the formation of these two 

 parts will be considered separately, as has been customary 

 among the embryologists. 



