256 



INVEETEBEATA 



CHAP. 



sac 



body muscles. The inner wall in this region becomes converted into 

 a mass of cells which are somewhat loosely arranged, their nuclei lose 

 their chromatin, and fat drops are deposited in the bodies of the cells. 

 This mass is the rudiment of the glistening white fat body so char- 

 acteristic of insects (/.&, Fig. 208). Some cells of this fat body get 

 loose in the coelomic sac and tend to choke it up. Where the outer 

 wall of the sac passes into the inner, at its upper margin, peculiar 

 cells called " cardioblasts " are differentiated ; they are large -cells 



with clear pale nuclei, and are so 

 named because they take part in the 

 formation of the wall of the heart. 

 The inner wall of the coelomic 

 clings to the yolk, and as it 

 downwards it forms here a 

 single epithelial layer which is 

 destined to give rise later to the 

 visceral muscles. From this layer a 

 sheet of cells is given off which 

 extends underneath the yolk towards 

 the mid- ventral line, thus completing 

 the investment of the gut with 

 visceral muscles. Below this point 

 the inner wall of the coelomic sac 

 passes outwards to join the rudiment 

 of the fat body. In that part of 

 its course it is somewhat thickened, 

 and is termed, for reasons to be 

 explained later, the genital ridge 

 (Fig. 204, B). As development pro- 

 ceeds the coelomic sacs extend farther 

 and farther upwards at the sides of 

 the yolk, carrying the cardioblasts 

 on their crests. 



Hirschler states that at the same 

 time the cavities of the coelomic 

 sacs open ventrally into the epineural sinus, owing to the breakdown 

 of the coelomic walls at this point. This statement is supported by 

 similar statements on the part of other authors who have worked 

 at the development of other species of Insecta; nevertheless it is 

 a statement which must be received with very great caution, because 

 it is at total variance with what we know of the fate of the coelom 

 in other groups of the animal kingdom. For this reason we doubt 

 it, and think it is probable that the statement is a mistake, founded 

 on a result which one is apt to get by using only paraffin wax for 

 embedding material. When the brittle and loosely connected cells of 

 a yolky embryo are penetrated by hot xylol and hot melted paraffin, 

 diffusion currents are set up which are apt to produce artificial 

 ruptures, but when the tissue is slowly infiltrated by cold solution 



Fig. 20i. — Two transverse sections 

 through embryos of Donacia crassipes 

 of different ages, to show the modi- 

 fications undergone by the coelomic 

 sacs. (After Hirschler.) 



A, younger stage. B, older stage, 

 wandering blood cells ; coef coelomic cavity ; 

 ep, epineural sinus ; geii.r, genital ridge ; it.6, 

 neuroblast in process of division ; v.n.c, 

 ventral nerve cord. (In B the section passes 

 through a pair of ganglia.) 



