THE CCELOM A.\D DORSAL VESSEL. 87 



The cells of the fat bodies increase rapidly in size with the 

 growth of the larva, and, except in very young larvae, do not 

 appear to increase in number. 



The fat body is undoubtedly the principal store of nutrient 

 material for the development of the imago. 



The Blood of the larva consists of an opalescent spontaneously 

 coagulable fluid, and has a large number of amoeboid corpuscles 

 5'* to 6" in diameter (Fig. 17). It permeates all the tissue 

 spaces of the ccelom, but there are several distinct blood 

 sinuses through which the direction of the blood stream 

 appears to be constant. The largest are the great ventral and 

 the pericardial sinuses. 



The Great Ventral Sinus commences between the imaginal 

 discs of the head, and extends forwards to the pharynx and 

 maxillae as the cephalo-pharyngeal sinus, and backwards sur- 

 rounding the neuroblast and the alimentary canal as far as the 

 posterior border of the 12th somite, where it is lost in spongy 

 tissue, through which the blood ascends to the pericardial 

 sinus. 



The Dorsal Vessel (Figs. 18 and 19). There is no organ in 

 the larva the study of which presents such difficulties as the 

 dorsal vessel. It is a muscular tube which extends from the 

 posterior transverse tracheal trunk to the ring, to both of 

 which it is attached. As Weismann pointed out [2, p. 121], it 

 consists of three parts, which I shall distinguish as the ventricle, 

 the intermediary portion, and the aorta. 



The Ventricle (Fig. 10, dv) is ovoid, flattened from above 

 downwards and constricted at two points, so that it consists of 

 three chambers. It lies in the dorsal or pericardial sinus, and 

 is separated from the intestines by a partial septum — the 

 pericardial septum. 



The Pericardial Sinus lies immediately beneath the dorsal 

 integument of the nth, 12th, 13th, and 14th somites. The 

 ventricle only partially fills the cavity, which contains tufts of 

 fine tracheae and a quantity of lymphoid tissue, indistinguishable 

 from the lymphoid tissue of vertebrates. This is especially 

 abundant on either side of the ventricle. 



