262 Miscellaneous. 



MISCELLANEOUS. 



On the Earliest Stajes in the Development of Sessile-ey-'.d dmstacea. 

 By M. Loris Koule. 



I HAVE had the houour of communicating to the Academy several 

 of the most important phenomena presented hy the sessile-eyed 

 Crustacea in the course of their embryogeny ; some more recent 

 investigations enable me to complete the knowledge already acquired 

 and to prepare a synthesis of the first stages of the development, 

 taking as types Asellus aquaticus and Porcellio scaber. 



The ovule is always rich in nutritive vitellus ; nevertheless the 

 bulk occupied by the latter varies accordirrg to the species. When 

 it is least in amount the fertilized ovum undergoes a total and radial 

 segmentation, the segments assuming the well-known form of cones, 

 of which the apes is turned towards the centre of the ovule and the 

 base towards the perij^hery ; on the contrary, when its quantity is 

 considerable, as in Porcellio for instance, this preliminan,- segmen- 

 tation is not manifested. But, whatever be the mode exhibited, 

 after the radial division when it exists, or from the moment that the 

 ovum is mature when it does not appear, the formative vitellus 

 (" vitellus evolutif) does not remain mingled with the nutritive 

 vitellus, but separates from it. This separation does not manifest 

 itself at the same time throughout the ovum ; it commences in a 

 zone which corresponds to the future anterior extremity of the 

 embryo. The formative vitellus forms in the first place in this 

 region a little cicatricle, which rapidly oiganizes itself into cells, to 

 which the nuclei are furnished by the conjugated nucleus, which 

 results from the fusion of the male and female pronucleus effected 

 in fertilization. Fresh quantities of formative vitellus then become 

 isolated from the nutritive vitellus and added to the cicatricle, in- 

 creasing its mass and dividing likewise into ceDs ; in this way the 

 cicatricle grows and gradually cnvelo])s the nutritive vitellus, advan- 

 cing with regularity from the zone which it occupied until it reaches 

 the pole diametrically opposite ; a cellular layer is extended by this 

 proceeding u])on the perijjher}' of the ovule, and finally surrounds it. 



Arrived at this stage of development, the embryo is constituted 

 by a layer of cells which surrounds a compact mass of nutritive 

 vitellus ; this layer is the blastuderm, which will give rise to the 

 three blastodermic layers. To this end the cells of the blastoderm 

 produce a large number of cellular elements, of which some pene- 

 trate into the nutritive vitellus, while the rest intercalate themselves 

 between the latter and the blastodermic layer ; the development 

 of the two kinds of cells is similar. Several of these elements, col- 

 lected in two groups placed at the sides of the embryo and not far 

 from the medio-ventraWine. arrange themselves in two symmetrical 

 layers which penetrate into the nutritive vitellus, converging towards 

 one another ; these two layers, separated from the time of their firet 

 appearance, represent the rudiments of the endoderm. Tlie other 

 elements do not arise in limited zones : they are jiroduoed by the 

 blastoderm throughout its entire extent, and give rLse to the meso- 



