GENERAL CONSIDERATIONS. 143 



after the blastopore has closed, probably becomes transformed direct 

 into the rudiment of the mid-gut. Moina, and perhaps also Lucifer 

 and Penaeus, seem to approach Cetochilus in this respect. On the 

 whole, the formation of the entoderm by invagination is fairly common 

 among the Crustacea. In other cases — Arthrostraca, Mi/sidae, Cuma- 

 cea, Cirripedia (?) — an invagination does not occur, the entoderm 

 appearing as a solid ingrowth of cells (Fig. 68 B, p. 136). 



Important and characteristic variations in the fate of the mass 

 of entoderm-cells are to be found in the development of Crustacean 

 eggs rich in yolk. In those processes by which the rudiment of 

 the mid-gut with its hepatic diverticula becomes differentiated, the 

 relation of the masses of food-yolk to this rudiment produces a 

 marked influence. At the beginning of development, after the 

 blastoderm has formed, the blastocoele appears filled with food-yolk. 

 As a consequence, when gastrulation commences, the nutritive yolk 

 lies outside the archenteric vesicle in the so-called primary coelom. 

 Later, however, the rudiment of the mid-gut is, as a rule, developed 

 in such a way that it contains within it the whole of the food-yolk. 

 A change has thus taken place in the relative position of the 

 rudiment of the entoderm and the yolk. This change may take 

 place in three different ways in the Crustacea, giving rise to three 

 different types of development of the mid-gut, which may be charac- 

 terised as follows : — 



I. Development of the mid-gut through the filtration of the 

 food-yolk, e.g., in Astacus (Fig. 63, p. 131). The food-yolk which 

 lies in the primary coelom is gradually taken up into the entoderm- 

 cells. When this process is completed, the secondary yolk-pyramids 

 having developed, the nuclei of the entoderm-cells shift to the 

 surface of the yolk, where, with the protoplasm surrounding them, 

 they form the epithelium of the mid-gut, within which the whole 

 mass of yolk is ultimately situated. The typical feature of this 

 process is that the archenteric cavity formed by invagination 

 persists throughout development, the lumen of this vesicle passing 

 into the lumen of the future mid-gut. The entoderm-cells, further, 

 never lose their epithelial continuity. The food-yolk, originally 

 lying outside the entoderm-vesicle, first enters the wall of this 

 vesicle and finally passes into its lumen. This method of forma- 

 tion of the mid -gut, of which Astacus is the only case known, 

 appears to stand quite alone ; it is of all the greater interest as 

 affording a key for the comprehension of the two other types of 

 development. 



