CRUSTACEA. 437 



that the heart originates from this mass. At the time when the heart can 

 first be made out and before it has begun to beat, it has the form of an 

 oval sack with delicate walls separated from the mesenteron by a layer of 

 splanchnic mesoblast. Its cavity is filled with a peculiar plasma which 

 also fills up the various cavities in the mesoblast. Around it a peri- 

 cardial sack is soon formed, and the walls of the heart become greatly 

 thickened. Four bands pass off from the heart, two dorsalwards which 

 become fixed to the integument, and two ventralwards. There is also a 

 median band of cells connecting the heart with the dorsal integument. 

 The main arteries arise as direct prolongations of the heart. Dohrn's 

 observations on Asellus greatly strengthen the view that the heart 

 originates from a solid mesoblastic mass, in that he was able to observe 

 the hollowing out of the mass in 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 

 (antenna?, appendages, etc.). The corpuscles are formed as free nuclei 

 and are primarily derived from the yolk, which at first freely communicates 

 witli the cavities of the appendages. 



Alimentary tract. In Astacus the formation of the mesenteron 

 by imagination., and the absorption of the yolk by the hypoblast cells, 

 have already been described. On the absorption of the yolk the me- 

 senteron 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 proctodseum 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 mesenteron ; 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 mesenterou 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 proctoda^um very early opens into the mesenteron, but the stomo- 

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

 completed. The proctodseum gives rise to the abdominal part of the intestine, 

 and the stomodseum to the oesophagus and stomach. The commencement 

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

 thickening of the epithelium. 



