OP ARTS AND SCIENCES. 75 



I'hypothese d'aprcs laquelle les materiaux destines a fournir h la 

 moitie droite du corps siegeraient dans I'un des hemispheres lateraux 

 de I'oeuf, tandis que I'hemisphere ovulaire gauche engeudrerait tons 

 les organes de la moitie gauche du corps." 



If our ideas of the promorphological relations of the ovum are 

 well founded, it will be seen that we have a very satisfactory founda- 

 tion for the opinion, first suggested by Balfour (Comp. Emb., II., 

 p. 312), that the neural surface is identical throughout the metazoa. 

 It is hardly necessary to add, that this view is in perfect accord with 

 the theory of concrescence before mentioned. Indeed, it is difficult 

 to see how one can hold to the former, and deny the latter. 



EXPLANATION OF FIGURES. 

 Figs. 1-5, Ctenolahrus. Fig. 6, Ps. ohlongus. 



All magnified '2S0 diameters. 



Fig. 1. Blastodisc seen from the inner surface. The Arabic nu- 

 merals give the order of the cleavage-planes ; the shading indicates 

 that portion of the floor of the marginal cells which rests on the 

 yolk, o, b, c, d, central cells ; cl, boundary of the cleavage-cavity ; 

 xy, plane of the vertical section seen in Fig. 2. 



Fig. 2. Transverse section of the 16-cell stage, in the plane indi- 

 cated by the dotted line (xi/) in Fig. 1. p, periblast; be, cleavage- 

 cavity. 



Fig. 3. Section of the blastodisc one hour after tlie 16-cell stage. 

 The marginal cells are shaded. 



Fig. 4. Two hours after the 16-cell stage, ep, epidermal layer. 



Fig. 5. Three hours after the 16-cell stage. The marginal cells 

 assuming the form of periblastic cells. 



Fig. 6. Transverse section at the time when the entodermic ring 

 appears, en, basis of the future entoderm. 



