GENERAL CONSIDERATIONS. 141 



soon dividing into four. The process in Caprella and Sunamphithoe differs 

 in so far that, in these genera, the rudiments of the hepatic tubes become 

 differentiated before the rest of the mid-gut. In this respect the above forms 

 resemble Oniscns. The mesoderm, on the contrary, is said to arise at a com- 

 paratively late stage in the rudiments of the individual limbs, by a kind of 

 delamination from the ectoderm (?). 



The Curaacea agree most nearly with the Isopoda in the formation 

 of the germ-layers, the position and form of the germ-band, and of 

 the dorsal organ. In them, discoidal cleavage gives rise first to 

 a rounded disc, which gradually grows over the surface of the 

 egg. Before, however, the blastoderm is completely formed, a pro- 

 liferation of cells at the centre of the disc takes place, leading 

 to a massing of the cells beneath the blastoderm. In these inner 

 cells, two layers, mesoderm and entoderm, are soon discernible. 

 These processes are very similar to those described by BOBRETZKY 

 for Oniscus. The two lower layers then spread along a band-like 

 area on the ventral side of the egg, the area in this way developing 

 into the embryonic rudiment or "germ-band." The latter is not 

 confined to the ventral side, for its anterior and posterior ends soon 

 spread over the dorsal parts of the egg. At the same time a mass 

 of cells, known as the "dorsal organ" (p. 150), develops on the dorsal 

 side. In all these ontogenetic processes the food-yolk apparently 

 takes no active part. At first, also, no cell-elements are perceptible 

 in the yolk, but after the germ-band has developed, isolated crescent- 

 shaped cells, each enclosing a granule of yolk, appear within it. 

 There are also quite isolated, large, finely granulated cells in the 

 food-yolk. These cells do not appear to assist in any way in the 

 development of the mid-gut, and their function is still uncertain. 

 In later stages (when the limbs develop) the food-yolk has been 

 observed to break up into large spheres (H. BLANC, No. 35). 



GK General Considerations. 



We have first to consider the position of the blastopore. If, in 

 this respect, we institute a comparison with allied groups of animals, 

 especially with the Annelida (see Vol. i., p. 265, the closing of the 

 blastopore in Eupomatus), we should similarly be inclined to regard 

 as the blastoporal region in the Crustacea the whole area lying 

 between the mouth and the anal aperture. Such an extension of 

 its position is indicated, however, in the ontogeny of only a very 

 few forms ; for example, in the fissure-like primitive mouth of 

 CetocJiilus, which closes from before backwards (GROBBEN, p. 121), 



