No. I.] CONTRIBUTION TO INSECT EMBRYOLOGY. 113 



developed mesodermic somites. When we stop to consider the 

 intimate union of the proto- and deutocerebral ganglia from 

 the time of their first appearance, we need entertain littl^ hope 

 of finding traces of a separation which existed, if indeed it 

 existed at all, in a very remotely ancestral period. 



VI. The Development of the Reproductive Organs 



IN the Insecta. 



I, TJie Gonads. 



The following account of the development of the sexual 

 organs is based almost exclusively on XipJiidium. Some 

 attention was devoted to the study of Blatta, but this form 

 proved to be so much less satisfactory and to depart so little 

 from the Xiphidmm type, that it was abandoned. 



Before passing to a description of the sexual organs and 

 their ducts, it will be necessary briefly to consider the meso- 

 dermal somites, since the history of the organs tinder consid- 

 eration is intimately bound up with the history of the middle 

 germ-layer. The mesoderm of XipJiidmm, like that of other 

 insects, is coextensive with the blastopore and hence reaches 

 from the region of the definitive mouth to the region of the 

 definitive anus. At first a continuous cell-layer, it soon splits 

 up into segmental masses as metamerism sets in. These are 

 further divided in the median ventral line so that each segment 

 has a pair of mesoderm blocks. Each of these acquires a 

 cavity and the somites are established. 



The appendages are from the time of their first appearance 

 intimately connected with the somites, since each of the latter 

 sends a hollow diverticulum into the appendage of the corre- 

 sponding half of the body. All the somites are fully formed 

 when the embryo has reached Stage F. There are then eight- 

 een pairs in all. The most anterior pair occupies the deuto- 

 cerebral segment and sends hollow diverticula into the 

 antenna. The walls of these somites are much thinner than 

 those of succeeding pairs and, curiously enough, persist much 

 longer. The pair in the tritocerebral segment is very small 



