526 DEVELOPMENT OF ORGANS. 



the living embryo (cf. the development of the heart in Spiders). Some of the 

 central cells (nuclei, Dohrn) become blood corpuscles. The formation of 

 these is not, according to Dohrn, confined to the heart, but takes place in 

 situ in all the parts of the body (antennae, appendages, etc.). The corpuscles 

 are formed as free nuclei and are primarily derived from the yolk, which at 

 first freely communicates with the cavities of the appendages. 



Alimentary tract. In Astacus the formation of the mesenteron by 

 invagination, and the absorption of the yolk by the hypoblast cells, have 

 already been described. On the absorption of the yolk the mesenteron has 

 the form of a sack, the walls of which are formed of immensely long cells 

 the yolk pyramids at the base of which the nucleus is placed (fig. 238 B). 

 This sack gives rise both to the portion of the alimentary canal between the 

 abdomen and the stomach and to the liver. The epithelial wall of both of 

 these parts is formed by the outermost portions of the pyramids with the 

 nuclei and protoplasm becoming separated off from the yolk as a layer of 

 flat epithelial cells. The yolk then breaks up and forms a mass of nutritive 

 material filling up the cavity of the mesenteron. 



The differentiation both of the liver and alimentary tract proper first 

 takes place on the ventral side, and commences close to the point where the 

 proctodasum ends, and extends forward from this point. A layer of epithelial 

 cells is thus formed on the ventral side of the mesenteron which very soon 

 becomes raised into a series of longitudinal folds, one of which in the 

 middle line is very conspicuous. The median fold eventually, by uniting 

 with a corresponding fold on the dorsal side, gives rise to the true mesente- 

 ron ; while the lateral folds form parallel hepatic cylinders, which in front 

 are not constricted off from the alimentary tract. The lateral parts of the 

 dorsal side of the mesenteron similarly give rise to hepatic cylinders. The 

 yolk pyramids of the anterior part of the mesenteron, which projects 

 forwards as a pair of diverticula on each side to the level of the stomach, are 

 not converted into hepatic cylinders till after the larva is hatched. 



The proctodasum very early opens into the mesenteron, but the stomo- 

 daeum remains closed till the differentiation of the mid-gut is nearly 

 completed. The proctodaeum gives rise to the abdominal part of the intes- 

 tine, and the stomodaeum to the oesophagus and stomach. The commence- 

 ment of the masticatory apparatus in the latter appears very early as a 

 dorsal thickening of the epithelium. 



The primitive mesenteron in Palaemon differentiates itself into the 

 permanent mid-gut and liver in a manner generally similar to that in 

 Astacus, though the process is considerably less complicated. A distinct 

 layer of cells separates itself from the outer part of the yolk pyramids, 

 and gives rise to the glandular lining both of the mid-gut and of the liver. 

 The differentiation of this layer commences behind, and the mid-gut very 

 soon communicates freely with the proctodasum. The lateral parts of 

 the primitive mesenteron become constricted into four wings, two directed 

 forwards and two backwards ; these, after the yolk in them has become 

 absorbed, constitute the liver. The median part simply becomes the me- 



