336 Miscellaneous. 



in the body of the embryo, these cells are placed between the blasto- 

 derm and the eudoderm ; they multiply by karyokinesis, just like 

 those of the limbs. 



The middle layer is now constituted. The elements arise from 

 the blastoderm, which, after having provided for their genesis, 

 persists as the ectoderm on the surface of the body. Moreover, its 

 cells are distributed throughout the entire embrj'o between the 

 blastoderm and endoderm, are immersed in the deutoplasm, which 

 they devour little by little, and are accumulated in large numbers 

 in the rudiments of the feet. 



The mesoderm will next develop in the mesenchymatous fashion. 

 The mass of cells placed in each budding foot commences by acquiring 

 a central cavity, or sometimes two or three adjoining one another ; 

 the cells which surround this cavity separate from their neighbours 

 and become free in its interior. The whole of the elements of the 

 mass gradually become involved in this process of dissociation ; 

 they increase in length, collect into bands crossing one another in 

 different directions, and become transformed into muscle-fibres. 

 The result is the production, in the space limited by the ectoderm 

 of the limb, of a plexus of mesodermic elements ; the meshes of 

 this plexus are spaces filled with a liquid containing a few cells 

 which have not undei'gone transformation, and which become the 

 vascular sinuses of the appendage ; the plasma which fills them and 

 its cells represent the nutritive fluid. The fact that a little central 

 cavity is primitively present in each young ajipendage has caused 

 many embryogenists to admit the regular metameric division of the 

 ventral mesodermic bands, and that, not only for the Crustacea, but 

 also for the rest of the Arthropoda (excepting Peripatus^ which 

 appears to me to be wrongly included among the Arthropods). 

 There is nothing in this mode of development which is comparable 

 to the partitioning of the coelome of the Annelids and Vertebrates ; 

 the whole process stops at the development in the appendages, while 

 they aie still quite small, of clefts which are destined to become 

 blood-lacttnaj and of which the first arises almost at the centre of 

 the limb. 



Tlio mesoderm of the body also develops in a similar way ; its 

 elements, by devcmring the nutritive yolk, occasion the formation of 

 singular spaces, which communicate with one another and develop 

 into blood-lacuna3 ; one of the latter, however, surrounding the 

 intestine, becomes isolated from its neighbours and constittites the 

 }>eri-intestinal cavity. But before this separation is effected, a group 

 of mesodermic cells, sitttated above the proctodoeum, elongates and 

 accjuires a central cavity, which proceeds to unite with the meso- 

 dermic spaces : this hollow mass is the rudiment of the heart. 



To sum up our results. The mesoderm is produced by almost 

 the whole of the blastoderm, without the appearance of enterocoelic 

 rudiments or diverticula ; its elements develop by the mesenchy- 

 matous process ; the sole representative of a coelome is the ensemble 

 of the circulatory apparatus and the perivisceral cavities, which has 

 the value of a psetidocoele ; no portion of it undergoes metameriza- 

 tion such as is met with in the Annelids or Vertebrates. — C'omptes 

 Jhiuhif!, tome cxiii. no. 3 (20 juillet, 1801), ])p. 15'3 Io.'k 



