ORIGIN OF THE REPRODUCTIVE GLANDS 575 



Hydrophilus a dorsal band of the fat-body passes over the digestive 

 canal arising by direct transformation of the wall of the coelom-sacs. 

 But also the other portions of the fat-body, as the fat-body lobes 

 accompanying the tracheal system, are of undoubted mesodermal 

 origin. Heymons' observations on the cockroach (Phyllodromia) 

 agree with the foregoing view. In this insect at a very early period 

 certain cells in the wall of the coelom-sacs undergo a change, and 

 may be recognized as the rudiments of what are afterwards fat-body 

 tissues (Fig. 540, B and C, /). 



The reproductive organs Our knowledge of the mode of develop- 

 ment of the genital organs is in a less satisfactory state than that 

 of the other organs. It is now known that the rudiments of the 

 sexual glands belong to the mesoderm, and are developed from the 

 wall of the coslom-sacs. In the cockroach (Phyllodromia), the most 

 generalized of the winged insects, as Heymons has shown, in the 

 earlier stages of the embryo separate genital cells are already dis- 

 tinguished by their histologically different characters from the other 

 mesodermal cells. The genital cells are larger and show a feebly 

 stained nucleus with a clear nucleolus. These genital cells, which 

 are transformed normal mesodermal cells, lie originally within the 

 mesoderm layer or on the surface of this layer turned towards the 

 yolk, on the edge of the segments. After the complete formation 

 of the coelom-sacs we find them (Fig. 549, gz) in the dissepiments 

 which separate the successive coelom-sacs from one another. Here 

 new genital cells are constantly formed through the transforma- 

 tion of mesoderm cells. The development of the genital cells takes 

 place in the 2d to the 7th abdominal segments. 



Afterwards the genital cells pass into the interior of the coelom- 

 sacs, and soon pass to the dorsal wall of the same (Fig. 540, A, gz) 

 and enter between the cells of this wall. The coelom-sacs (c) show 

 in cross-section in this stage a triangular outline, so that we can 

 distinguish a dorsal, lateral, and median wall. The dorsal wall lies 

 next to the surface of the yolk, and afterwards gives rise by separa- 

 tion or splitting to the splanchnic mesoderm (Fig. 544, sp~), while 

 from its remains the terminal thread-plate (e/) originates. The 

 lateral wall, which is turned towards the ectoderm of the primitive 

 band, is intimately concerned in the formation of the somatic layer 

 (Fig. 540, C, so) of the mesoderm. Out of what remains arises the 

 pericardial septum (Fig. 544, ps). 



When the genital cells have entered into the dorsal wall of the 

 primitive segments, they are already so numerous that they form a 

 continuous series extending from before backward. The genital 



