334 Miscellaneous. 



8tas;e the nutritive yolk is surrounded by a sini^jle layer of blasto- 

 dermal elements. 



The blastoderm then proliferates in several regions and upon the 

 inner surface. One of these regions, which occupies the future 

 median and ventral line of the embryo, extends from the anterior to 

 the posterior extremity of the ovum ; a projecting band arises, 

 Avhich advances into the yolk, and rapidly divides into two parallel 

 and adjoining zones. This parallel band will give origin to the 

 nervous centres ; it is interrupted beneath the anterior pole of the 

 body, at a spot where the stomodceumm appears ; divided in this way, 

 its anterior portion constitutes the rudiment of the brain and its 

 posterior section that of the ventral cord. 



At the moment when the first indications of the nervous centres 

 are seen, the blastodermal elements multiply in two regions situate 

 upon the sides of the embryo, a little behind the cerebral rudiment 

 and on both sides of the median line. Each of these tracts soon 

 exhibits, beneath the blastoderm, a layer of cells which extends in 

 three directions — above, below, and behind. When the extension 

 in the two former directions has arrived at a certain point it stops, 

 and the layer of cells buries itself horizontally, by its upper and 

 lower edges, in the nutritive yolk, upon which it acts like a punch. 

 This new extension ceases when the two edges reach the median 

 line ; they then bend inwards, and, continuing to grow, approach 

 one another until they meet and unite. Each layer has thus formed 

 a tube, which occupies the greater portion of the corresponding 

 half of the body of the embryo, and the cavity of which, closing 

 behind, contains the nutritive yolk which it has imprisoned during 

 its development. These two tubes are the rudiment of the organ 

 erroneously termed the Crustacean Uver ; this organ, bounded by 

 the cndoderm of which we have just traced the mode of formation, 

 should be regarded as the enteron of these animals ; its functions, 

 moreover, notably in the case of the lower Crustacea, are nutritive 

 rather than glandular. 



Apart from the liver, the remainder of the alimentary canal is 

 derived from two opposite blastodermic invaginations, one of which 

 is inferior and somewhat ventral, the other superior and slightly 

 dorsal. The two depressions sink into the yolk in order to meet 

 one another; they first touch, then fuse, and the region of their 

 juncture unites with the liver at two points. The anterior or sto- 

 modeai invagination produces the oesophagus and stomach, while the 

 posterior or proctodeal gives rise to the intestine. 



The mesoderm arises while these different processes are in 

 progress. This layer is produced by the elements of the blasto- 

 derm ; the majority of these divide into segments, the external of 

 which continues to form part of the blastodermal layer, while the 

 internal penetrates into the yolk. The latter divides in its turn 

 into several other cells, and, the same thing happening for the whole 

 of the blastoderm, the aggregate of these elements constitutes the 

 mesoderm. The principal zones of proliferation are situated on the 

 Ventral face of the body, at the base of the limbs ; they are conse- 

 quently two in number, situated one on each side of the median 



