144 CRUSTACEA. 



II. Development of the mid-gut by the interpenetration of the 

 food-yolk, e.g., Palaevion (Fig. 64, p. 132). The cells of the gastiula 

 that arise by invagination here very soon lose their epithelial con- 

 tinuity, the archenteric vesicle itself apparently disappearing in this 

 process. The entodermal elements, in tlie form of amoeboid wandering 

 cells, pass separately into the yolk and traverse it, finally becoming 

 arranged at its surface to form the epithelium of the mid-gut. Tt 

 is evident that in this case also the direction in which the entoderni- 

 cells move is the same as in Type I. The difference between the 

 two types consists in the fact that in the type we are considering, 

 the wall of the archenteron temporarily loses its continuity. So as 

 to deduce this type from the last, we should have to imagine that, 

 in consequence of the small number of the entoderm-cells and tlie 

 greater volume of the yolk, the intervals between the entoderm-cells, 

 as they shifted apart during the increase in size of the archenteron, 

 became so great that the epithelium could not retain its continuity. 

 The temporary independelice of the entodermal Avandering ce)ls 

 appears, however, to have afforded a simplification of the process 

 of development, by means of wliich the commencement of immi- 

 gration into the yolk was made possible at an earlier stage. This 

 second type of development seems to be very common among the 

 Crustacea. It is found in most Decapoda, and apparently also in 

 the eggs of many Entomostraca which abound in yolk (those of 

 the Cirripedia, Copepoda, Cladocera). We shall also find it in 

 many other groups of Arthropoda, e.g., Li)nuht,% the Araneae, etc. 

 During the process of wandering through the food-yolk, the scattered 

 entoderm-cells frequently give rise to a secondary demarcation of 

 tlie yolk into the cell-areas ; this has been called yolk-cleavage, but 

 has, of course, no connection with actual cleavage, which must be 

 regarded as concluded when the blastoderm has developed. 



III. Development of the mid-gut by circumcrescence of the 

 food-yolk, e.g., Mysis and Ligia (Fig. 66, p. 135). Here, as in 

 the preceding type, individual cells become separated from the mass 

 of entoderm-cells (which, in forms belonging to this type, always 

 arises by solid ingrowth) ; these separated cells enter the yolk and 

 become scattered within it. They do not take any part in the later 

 structure of the mid-gut ; as vitelloj)hags they assimilate food-yolk 

 and either become disintegrated later, or else, perhaps, persist as 

 blood-corpuscles. The chief mass of entoderm-cells, however, does 

 not join in this immigration, but remains near its place of origin at 

 the surface of the yolk, and changes later into two paired, disc-shaped 



