ARTHL03TRACA AND CUMACEA. 139 



is not very clear. The two rudiments unite in the anterior part of 

 the embryo, and there form the wall of the mid-gut; two channel- 

 like processes also grow out posteriorly ; these are open and concave 

 towards the yolk, and convex externally, and are closely applied to 

 the germ-band ; they are the rudiments of the primary hepatic tubes, 

 which only at a later stage divide into four by longitudinal constric- 

 tion. The rudiment of the mid-gut is covered posteriorly and 

 dorsally by the mass of yolk. This latter, as the entoderm-cells 

 increase in number by division, becomes completely surrounded by 

 the epithelium of the mid-gut, and in this way this rudiment which, 

 in the younger stages, was open dorsally, becomes gradually closed. 

 The mid-gut rudiment in Ligia, as in Porcellio, is fairly large, 

 whereas in Oniscus it seems to be limited to the hepatic outgrowths 

 and the parts of the intestine lying immediately adjacent to their 

 openings. 



The method of formation of the germ-layers which has been described by 

 Nusbaum for Ligia may, perhaps, be fairly common among the Isopoda, 

 although our knowledge of these processes is still too incomplete to enable 

 us to form a decided opinion on the subject. One of the forms in which the 

 origin of the germ-layers is best known is Oniscus. Here also the blastoderm 

 is said (Boeretzky, No. 80) to arise by discoidal cleavage, although, perhaps, it 

 may belong to our Type III. b (p. 115).* That region of the surface at which the 

 blastoderm first appears is here also, as in Mysis and Ligia, indicated, at a later 

 stage, by a rounded thickening, the germ-disc. This corresponds with the future 

 ventral surface of the embryo, and yields the germ-band. The formation of 

 the germ-layers (as in Ligia, but without previous demarcation of the meso- 

 dermal and entodermal areas) is commenced by an immigration of cells at the 

 centre of the germ-disc. The gastrula-invagination is here replaced by a simple 

 ingrowth of cells, by means of which the germ -disc becomes multilaminar. 

 While its outer layer is transformed into the ectoderm of the germ-band, the 

 inner layer yields the mesoderm and the entoderm. Bobeetzky (No. 80), 

 to whom we owe the first comprehensive description of the development of 

 Oniscus, observed that from the inner layer single cells wander into the food- 

 yolk, traverse it, and by increasing in number within it, bring about by a 

 kind of secondary cleavage the disintegration of the yolk (Fig. 71, hy). These 

 cells are said to represent the entoderm, and later, like the cells in the yolk 

 of Palaemon, to give rise to the mid-gut (especially to the liver). The cells 

 of the intermediate layer which remain close to the germ -disc are, on the 

 contrary, said to represent the mesoderm (Fig. 71, m). Nusbaum has recently 



* This view is also in agreement with the observations of Roule (No. 92), 

 although these latter are far from clear. According to this author, a superficial 

 layer of cells forms in Porcellio; this increases by the addition of new protoplasmic 

 elements from the yolk. The nuclei in this layer of cells are, however, said to 

 arise spontaneously. This blastoderm, which Roule calls an ectoderm, first 

 appears in the anterior part of the embryo, spreads back from here over the 

 ventral surface, and finally also spreads over the dorsal surface. The inner 

 mass of food-yolk, enclosed by this cell-layer, is assumed by Roule to be the 

 meso-entoderm. 



