Miscellaneous. 335 



line. The mesodcrmic cells are nourished at the expense of the 

 nutritive yolk which surrounds them ; they develop in the typical 

 mesenchymatous fashion, and the cavities which arise between 

 them to form the vascular canals are at their commencement little 

 confluent lacunre of irregular outline. None of these cavities can 

 be considered as corresponding, whether in its mode of development 

 or its origin, to the mesodermal zoonites of the Annelids. 



The blastoderm provides for these different proliferations without 

 losing the appearance of a simple epithelial layer surrounding the 

 nutritive yolk ; it retains this condition after the rudiments of the 

 mesoderm with those of the eudoderm have arisen at its expense and 

 separated from it ; it then represents the ectoderm. — Comixes 

 Eendus, tome cxii. no. 25 (22 juin, 1891), pp. 14G0-1462. 



On the Development of the Mesoderm of Crustacea, and on that of the 

 Organs derived from it. By M. Louis Rotile. 



I have shown in a former note (June 1891) *, on the basis of the 

 embryonic stages of Forcellio scaher, Latr., the process of the forma- 

 tion of the endoderm ; the layer is produced from a pair of rudiments 

 arising from two symmetrical regions of the anterior portion of the 

 blastoderm. The mesoderm also has the same origin, with this 

 difference, however, that the mode of development is much less 

 regular. 



My observations have been conducted upon Porcellio scaber and 

 Palcenwn serratus, Fabr. At the moment when the cells of the blasto- 

 derm are multiplying in the median ventral line for the production 

 of the nervous centres, and on the sides of the anterior extremity of 

 the body to give rise to the rudiments of the endoderm, two new 

 zones of proliferation appear, one on either side of the ventral nervous 

 band. The different regions of each zone are not perfectly similar ; 

 some, separated by equal distances, are thicker than others, and 

 raise up the blastoderm which covers them and from which they have 

 arisen ; these elevated spots are the rudiment.s of the limbs. The 

 blastoderm left at the periphery will become the ectoderm of these 

 appendages ; the central mass of cells represents the mesoderm ; the 

 cells of this mass become transformed into muscle-fibres in the way 

 which I have described in a previous note (' Comptes Rendus,' 

 January 1891). 



An analogous multiplication of cells takes place throughout the 

 entire blastoderm, except in those regions which furnish the rudi- 

 ments of the nerve-centres and of the endoderm, only the process 

 is less vigorous ; its effect is to produce the elements which penetrate 

 into the yolk lying beneath the blastoderm, and destroy it little by 

 little by feeding upon the nutritive materials which it contains. 

 These elements correspond to the vitelline cells of authors, as to 

 which opinions have been so numerous and so contradictory ; they 

 all arise from the blastoderm alone, and are destined to form the 

 mesoderm of the body, without there being any differences of deve- 

 lopment between them or ground for distinguishing between a 

 primary and secondary mesoderm. Receiving their proper situation 

 * yfilp supn). 



