CRUSTACEA. 439 



that the later growth of the proctodaeum takes place at the expense of the 

 yolk cells. 



The liver sacks become filled with a granular material without a trace 

 of cells ; their posterior wall is continuous with the yolk cells, and their 

 anterior lies close behind the stomach. The proctodseum continually 

 grows forwards till it approaches close to the stomodseum, and the two 

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

 with the proctodseum. 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 stoinodseum, to form 

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

 and meseiiteron together, though the digestive fluids are 110 doubt mainly 

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

 dseum and stomodseum at first meet each other without communicating, 

 but before long the partition between the two is broken through. 



In Cymothoa (Bullar, No. 499) the proctodseurn and stomodseum 

 .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 (1) 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 csecal tubes freely open to the yolk. 

 The stomodsenm soon reaches its full length, but the proctodseum grows 

 forwards above the yolk till it meets the stomodpeum. 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 

 proctodaaum and stomodfeum. The yolk sack of Cymothoa no doubt 

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

 any part of the apparent proctodseum 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 meseiiteron 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 meseiiteron 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 meseiiteron become the liver. In Deca- 

 pods 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 meseiiteron not having a distinct cavity, the differen- 

 tiation of it, which proceeds forwards as in Decapods, appears simply like 

 a prolongation forwards of the proctodseum, the cells for the prolongation 



