3oG MisceUaneous. 



in the body of the cmhrvo, tlicse cells are placed between the blasto- 

 (lerra and Ibeendodcrm; they multiply by karyokinesis, ju>t like 

 those of the limbs. 



The middle layer is now constituted. The elements arise from 

 the blastoderm, which, after havinj^ provided for their genesis, 

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

 cells are distributed througliout the entire embryo between the 

 blastoderm and endoderm, arc 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. 

 Tlio mass of cells ])laced in each budding foot commences by acquiring 

 a ccntial 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 iu 

 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 ple.xus of mesodermic elements ; the meshes of 

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

 which have not undergone 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 jmmitively present in each young appendage has caused 

 many embryogenists to admit the regular metameric division of the 

 ventral mesodermic l)ands, and that, not only for the Crustacea, but 

 also for the rest of the Arthropoda (excepting Ptrijiatnx, which 

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

 There is nothing in this mode of development which is comj)arable 

 to the ])artitioning of the ccelome of the Annelids and Verlelirates : 

 the whole jirocess stops at the development in the ap])endages, while 

 they are still (piite small, of clefts which are destined to become 

 blood-lacun;e and of which the first arises almost at the centre of 

 the limb. 



The mcsodeim of the body also devtlojis in a similar way ; its 

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

 singtilar spaces, which communicate with one another and develop 

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

 intestine, becomes isolated from its neighbours and constitutes the 

 ])eri-intestinal cavity, lint before this sej)aration is eflected, a group 

 of mesodermic cells, situated above the proctodoeum, elongates and 

 accjuires a central cavity, which jtrocceds to unite with tlio 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 divert ioila ; its elenuMits develop by the niesenchy- 

 matous ])roces3 ; the sole representative of a ca^lome is the ensemble 

 of the circulatory ap])aratus and tlie perivisceral cavities, which has 

 the value of a pseudoc(rle ; no portion of it undergoes mctamoriza- 

 tion such as is met with in tlie Annelids or Vertebrates. — ('mtip/is 

 ]{fiii/iis. tome cxiii. no. 'A (L'i» juillet, IS!t]), jiji. l.">;; ].")."). 



