680 WILLIAM PATTEN. 



Soon after their formation the neural cords become trans- 

 versely divided into ganglia. Meantime the median furrow 

 has increased in depth, forming an invaginated portion of the 

 ectoderm which unites transversely each pair of ganglia. This 

 median portion of the nervous system gradually disappears. 

 Whether it actually forms the cross commissures, as claimed 

 by Hatschek, is still an open question. While my own 

 observations have not confirmed those of Hatschek, still, 

 nothing was found that actually militates against his theory. 



If we examine longitudinal sections of the embryo after the 

 middle cord has disappeared (PI. XXXVIc, fig. 37), we shall ob- 

 serve TWO granular spots or commissures on the dorsal side 

 of each ganglion. These spots increase in extent, and at length 

 fuse with corresponding tracts in the opposite half of the 

 ganglion, thus uniting the lateral halves of each pair of ganglia 

 by TWO cross commissures (PI. XXXVIc, figs. 37 and 38). 

 When the embryo is ready to hatch these double commissures 

 disappear ; whether they appear again in the adults of this insect 

 I am not able to say. Such commissures, however, are present 

 in the adult crab, and certain insects — Locusta — according to 

 Leydig; but whether they are of the same nature as those seen 

 in embryo Phryganids, or whether they are secondary modifi- 

 cations of these embryonic conditions, is a question that we are 

 not in a position as yet to answer. At all events, I have not 

 been able to find a description of such a condition in either 

 insect or crustacean embryos. 



Coincident with the ectodermic invagination joining the 

 lateral halves of each ganglion, transverse invaginations are 

 formed between the successive ganglia themselves (PI. XXXVIc, 

 fig. 37). Here also Hatschek asserts that the infolded cells 

 give rise to the long commissures. When such an invagination 

 has once been proved to exist, as it undoubtedly does, one does 

 not have to go far for an explanation ; in fact, that offered by 

 Hatschek is very plausible, and the only one at hand. But 

 this is one of the cases where it is extremely easy to find a 

 probable explanation, but very difficult indeed to prove that 

 it is really the case. In other words, we lack the positive 



