I 



437 



a big karyosome, and with clear hyaline cytoplasm. The 

 glandular sac consists of about twelve very large, somewhat 

 flattened cells, enclosing a large lumen. The cells are as 

 much as 23jui in diameter and have large granular nuclei 

 lljji in diameter. 



The hepatic caeca (fig. 10), which are usually falsely 

 regarded as malpighian tubules, are composed of a number 

 of very large rounded cells, arranged alternately in pairs; 

 their union is such that a considerable space is left by the 

 incomplete fusion of a cell with the one opposite it, while the 

 fusion of cells with those behind them is always complete. 

 The lumen is lined with myriads of minute cilia (figs. 10, -, ^ 

 146), wl^ose movement drives the secretion backwards (or 

 forwards, in the third caecum) into the intestine. The indi- 

 vidual cells measure about 17/x in diameter and have a 

 remarkable resemblance to the cells of the fat-body. The 

 nuclei are very large and heavily granular, and the cytoplasm 

 faintly granular, and already at this early stage slightly 

 vacuolated. • 



The hepatic caeca appear to be formed in the embryo as 

 outgrowths from the midgut; this is indicated by the fact 

 that in the first instar the third (posterior) caecum is present 

 as a short, solid, robust projection from the rear of the mid- 

 gut, and that it is only later that it acquires its ciliated 

 lumen. 



(3) The Post-emhryonic Development of the ^Intestine. 



The feeding period of the larva (i.e., about the first three 

 days of larval life) is characterized by the completion of 

 differentiation of the cells of the first instar, by a great growth 

 in cell size, and by a corresponding total absence of cell divi- 

 sion, except in the case of those cells which constitut-ed the 

 ''imaginal tissues" of the larval intestine, viz., (a) the 

 oesophageal imaginal ring, surrounding the posterior part of 

 the foregut; (h) the small "replacing cells," as I shall 

 designate them here, which lay scattered about at the bases 

 of the large cells of the midgut; and fc) the imaginal disc 

 at the ant-erior extremity of the rectum. 



The visible differentiation is not very marked ; it con- 

 cerns mostly the intestinal muscle cells which, though already 

 functioning, have not yet adopted a striated appearance ; 

 but before the end of the second instar this is alw^ays visible. 



(A) The Metamorphosis of the Foregut. 



During larval life there is a great increase in the size of 

 the muscle and epithelial cells of the foregut; at each moult 

 the cuticle is shed and secreted anew. 



