February 1892. J 



PSYCHE. 



219 



were first seen by Patten in Acilius. 1 

 They are also present, as I have been 

 able to make out, in Blatta, Xiphidium 

 and Dytiscus. With the proper methods 

 Graber would also probably have found 

 them in Stenobothrus. This second 

 pair of segmental invaginations, which 

 Patten took to represent a second pair 

 of tracheal ingrowths, are supposed by 

 Graber to be the formative centers of 

 the oenocyte clusters. He admits, how- 

 ever, that a delamination of the area 

 surrounding each pit contributes largely 

 to their formation. My own observa- 

 tions lead me to believe that the invagi- 

 nations are very weak and transient and 

 that they contribute very few, if any 

 elements to the clusters ; most of the 

 oenocytes originating by delamination 

 and immigration from a considerable 

 area just caudad to the stigmata. In 

 Lepidoptera this area is more extensive 

 than it is in the Orthoptera. 



In nearly mature Xiphidium embryos 

 the oenocyte clusters may be seen shin- 

 ing through the hypodermis much as I 

 have represented them in Fig. 2. Thev 

 form eight bands running along the 

 pleural wall just back of and alternating 

 with the stigmata. 



Now Graber maintains that the fat- 

 body, at least in part, arises from these 

 oenocyte clusters. But a section through 

 a young Blatta embryo (Fig. 3) shows 

 most conclusively that this is not the 

 case. At may be seen the oenocytes, 

 still forming a part of the ectoderm v 

 from which they have differentiated, 



1 On the origin of Vertebrates from Arachnids. 

 Quart, journ. micr. sci., vol.31, pt. iii, new ser. p. 317- 

 37S. 1890. 



while the fat-body e is simply a thick- 

 ened portion of the inner coelomic wall. 

 The thickening is largely due to an ac- 

 cumulation of fat-vacuoles in the cvto- 

 plasm of the mesoderm-cells. Were 

 Graber correct in his assumption we 

 ought either to find no adipose tissue in 

 the embryo outside of the eight trachiger- 

 ous abdominal segments or be able 

 to show that the oenocytes migrate into 

 the head, thorax and terminal abdom- 

 inal segments and there form the fat- 

 body — since fat-tissue is developed in 

 all these regions of the bodv. But al- 

 though some of the oenocytes do later 

 on migrate into the metathorax and 

 perhaps even into the mesothorax, they 

 never occur in the head. Moreover, 

 long before any migration takes place, 

 thickenings of the coelomic wall, sim- 

 ilar to that in the figure, are found giv- 

 ing rise to the fat-body in the thorax, 

 gnathitic segments and also in the ter- 

 minal segments of the abdomen. Fur- 

 thermore, the oenocytes, so far as I have 

 been able to observe, are always per- 

 fectly distinct from the fat-body, never 

 contain fat-vacuoles, and never divide 

 after they are once differentiated from 

 the ectoderm during embryonic life. 

 Their number is therefore subject to no 

 increase during the growth of the ani- 

 mal. They are, as TichomirofF claimed, 

 a series of organs sui generis. Although 

 they certainly resemble the blood-cor- 

 puscles in some insects, they are always 

 much larger and seem not to be amoe- 

 boid. They are never seen constricting, 

 or exhibiting any appearance of giving 

 rise to the blood-cells. It follows then 



