Euphausiidæ. 2 1 



as to give the ovum, when viewed laterally, a somewhat 

 bell-shaped appearance, and through its walls the dark- 

 coloured mesoderm may now be easily traced, at the same 

 time indicating the extent of the gastral hollow. The divi- 

 sion of cells in the ectodermal layer is still going on very 

 rapidly, and this layer is therefore at last composed 

 of a very large number of densely crowded and somewhat 

 flattened cells, in which the nuclei now appear very distinct- 

 ly defined. Even when the embryo has been distinctly 

 formed, this process seems to continue, the geminous 

 arrangement of the nuclei indicating their immediate origin 

 from a single one (see fig. 13). 



V. 

 Formation of the embryo* 



The first indication of the embryo consists in a slight 

 transformation of the gastrula, whereby a median part and 

 2 lateral bulgings may be distinguished. The median part 

 represents the body of the embryo itself, the lateral bulgings 

 the fundament, from which the 3 pairs of Nauplian limbs 

 are developed. At first these lateral bulgings are only very 

 slight; but soon they become more distinct, and, on a closer 

 examination, are found each to be divided into 3 successive 

 transversal eminences, the first of which is quite simple, 

 whereas the other 2 are slightly bilobed at one end 

 (see PI. 1, figs. 10, 11). It is now easy to decide, which is 

 the anterior and which the posterior extremity of the embryo, as 

 also which face is dorsal, and which ventral. The extremity 

 where the simple transversal eminence occurs, is the anterior, 

 and the face bounded by the bilobed extremities of the 



