528 DEVELOPMENT OF ORGANS. 



liver sacks, now united into one at their base, become directly continuous 

 with the proctodaeum. By the stage when this junction is effected the yolk 

 cells have completely disappeared. It seems then that in Oniscus the yolk 

 cells (hypoblast) are mainly employed in giving rise to the walls of the 

 liver ; but that they probably also supply the material for the later growth 

 of the apparent proctodaeum. It becomes therefore necessary to conclude 

 that the latter, which might seem, together with the stomodaeum, to form 

 the whole alimentary tract, does in reality correspond to the proctodaeum 

 and mesenteron together, though the digestive fluids are no doubt mainly 

 secreted not in the mesenteron but in the hepatic diverticula. The procto- 

 dasum and stomodaeum at first meet each other without communicating, but 

 before long the partition between the two is broken through. 



In Cymothoa (Bullar, No. 499) the proctodaeum and stomodaeum 

 develop in the same manner as in Oniscus, but the hypoblast has quite 

 a different form. The main mass of the yolk, which is much greater than 

 in Oniscus, is not contained in definite yolk cells, but the hypoblast is 

 represented by (i) two solid masses of cells, derived apparently from the 

 inner layer of blastoderm cells, which give rise to the liver ; and (2) by a 

 membrane enclosing the yolk in which nuclei are present. 



The two hepatic masses lie on the surface of the yolk, and each of them 

 becomes divided into three short caecal tubes freely open to the yolk. 

 The stomodaeum soon reaches its full length, but the proctodaeum grows 

 forwards above the yolk till it meets the stomodaeum. By the time this 

 takes place the liver caeca have grown into three large tubes filled with 

 fluid, and provided with a muscular wall. They now lie above the yolk, 

 and no longer communicate directly with the cavity of the yolk sack, 

 but open together with the yolk sack into the point of junction of the 

 proctodaeum and stomodaeum. The yolk sack of Cymothoa no doubt 

 represents part of the mesenteron, but there is no evidence in favour of 

 any part of the apparent proctodaeum representing it also, though it is 

 quite possible that it may do so. The relations of the yolk sack and hepatic 

 diverticula in Cymothoa appear to hold good for Asellus and probably for 

 most Isopoda. 



The differences between the Decapods and Isopods in the development 

 of the mesenteron are not inconsiderable, but they are probably to be 

 explained by the relatively larger amount of food yolk in the latter forms. 

 The solid yolk in the Isopods on this view represents the primitive mesen- 

 teron of Decapods after the yolk has been absorbed by the hypoblast cells. 

 Starting from this standpoint we find that in both groups the lateral parts of 

 the mesenteron become the liver. In Decapods the middle part becomes 

 directly converted into the mid-gut, the differentiation of it commencing 

 behind and proceeding forwards. In the Isopods, owing to the mesenteron 

 not having a distinct cavity, the differentiation of it, which proceeds forwards 

 as in Decapods, appears simply like a prolongation forwards of the procto- 

 daeum, the cells for the prolongation being probably supplied from the yolk. 

 In Cymothoa the food yolk is so bulky that a special yolk sack is developed 



