54 WHEELER. [Vol. VIII. 



But it would be out of place to consider the widest bearings 

 of Patten's hypothesis in this paper since I am concerned with 

 it only in so far as it bears on the germ-layers of insects. 

 Starting with the assumption that the blastopore is confined 

 to the mouth, he attempts to show that the median furrow is a 

 purely secondary structure. "That the median furrow of in- 

 sects is merely an ontogenetic adaptation is sufficiently evident 

 from the fact that it may be present or absent in closely 

 related forms." This, however, is not the case. On the contrary 

 the furrow or a slight modification of it is, we have every rea- 

 son to suppose, universally present in the Insecta, at least in 

 the Pterygota, and this wide occurrence of the structure is one 

 of the surest indications of its high antiquity and phylogenetic 

 importance. 



In the latter part of his discussion Patten admits that there 

 are " structures in Arthropods which may represent remnants 

 of gastrulas. For example, if the mouth and oesophagus of 

 Arthropods is primitive — and there is no reason to sujDpose it 

 is secondarily acquired — then we must look for primitive 

 entoderm at its inner end. I have figured in ' Eyes of 

 Acilius,' at the very anterior end of the embryo, a great sac 

 of entoderm cells which probably arise by invagination, 

 although the process was not directly observed. The sac, 

 which soon opens outward by the oesophagus, afterwards 

 becomes solid, and finally is converted into two longitudinal 

 bands, one on either side, extending backwards to the middle 

 of the body, where they become continuous with similar bands 

 extending forwards from the posterior end of the embryo." 

 Patten admits that true entoderm is formed at two widely 

 separated regions of the body, but he implies that only the 

 anterior centre is comparable to the entoderm of other animals, 

 the posterior centre being a new and purely adaptive formation. 

 It is just here that his theory appears to me to fail, since it 

 does not explain why the oral and anal centres should resemble 

 each other so very closely in origin, method of growth and 

 histological structure. 



