THE MUSCLES ; THE FAT-BODY AXD CCELOM. 



85 



to regard this phase of development as the permanent condition 

 of any ancestral form.* It is, therefore, of interest to find in 

 the fat-body of the Cockroach an example of a solid, meso- 

 blastic, excretory organ, functional throughout life, but without 

 efferent duct. 





Fig. 38. Fat-body of Cockroach, cleared with turpentine. A, young tissue, with 

 distinct cell-boundaries and nuclei, a few cells towards the centre with dead 

 contents ; JS, older ditto, loaded with urates, the cell- walls much broken down, 

 and the nuclei gone ; tr, tracheal tubes. X 250. 



The fat-body is eminently a metabolic tissue, the seat of 

 active chemical change in the materials brought by the blood. 



tf 



Its respiratory needs are attested by the abundant air-tubes 

 which spread through it in all directions. 



The considerable bulk of the fat-body in the adult Cockroach 

 points to the unusual duration of the perfect Insect. It is 

 usually copious in full-fed larvae, but becomes used up in the 

 pupa- stage. 



Extensions of the fat-body surround the nervous chain, the 

 reproductive organs and other viscera, Sheets of the same 

 substance lie in the pericardial sinus on each side of the heart. 



The Coelom. 



The fat-body is in reality, as development shows, the irregular 

 cellular wall of the coelom, or peri visceral space. Through 

 this space courses the blood, flowing in no defined vessels, but 

 bathing all the walls and viscera. In other words, the fat-body 



*/ 



is an aggregation of little-altered mesoblast-cells, excavated by 

 the coelom, the rest of the mesoblast having gone to form the 



O o 



muscular layers of the body-wall and of the digestive tube. 



. , 



* Balfour, Embryology, Vol. II., p. 003. 



