142 CRUSTACEA. 



and in the forward extension of the point of ingrowth of the 

 entoderm in Ligia (Fig. 67, p. 136), between the two mesoderm- 

 germs. In by far the greater number of cases, the observations made 

 suggest that the blastopore corresponds in position to the posterior 

 end of the germ-band, and perhaps to the anal aperture which forms 

 later. We must here, however, bear in mind the extreme shortness 

 of the first embryonic area (germ-disc) in many Crustacea, and 

 imagine that as this short rudiment developed to form the longer 

 germ-band, elements which originally lay in the region of the closing 

 blastopore came, through the changes in position that take place 

 during growth, to lie more anteriorly, so that, perhaps, the blasto- 

 pore in the Crustacea ought to be regarded as having a greater 

 extension forward than is usually claimed for it. 



If we consider that the position of the future posterior end of 

 the body and the vegetative pole of the egg is indicated by the 

 closing of the blastopore, we should then (taking into account 

 the conditions in the Annelida) assume for the position of the 

 anterior end, the rudiment of which is distinctly indicated by the 

 development of the cephalic lobes (optic lobes), the diametrically 

 opposite point of the periphery of the egg. Such a position is 

 only, however, approximately gained by the cephalic lobes in a 

 few cases (e.g., Moina, Fig. 58 C, b, s, p. 124, and Cetochilus). In 

 eggs rich in yolk, the cephalic lobes and the blastopore belong to 

 the same hemisphere of the egg^ and lie more or less close to one 

 another (see Fig. 61, p. 129, where the anterior end of the body is 

 indicated by a star). It is thus evident that, within the egg, the 

 rudiment of the future ventral surface is much shorter than that 

 of the dorsal side, or, in other words, the dorsal side of the 

 embryo seems considerably swollen by the deposit of masses of 

 food-yolk, and is correspondingly retarded in its development. The 

 food-yolk sphere thus lies excentrically with relation to the longi- 

 tudinal axis of the developing embryo, being shifted dorsally. 

 Comparison with the Yertebrata, where the blastopore lies dorsally 

 and the food-yolk shifts to the ventral side, is in many respects 

 instructive (p. 118). 



When we come to consider the manner in which the germ-layers 

 develop, taking first the origin of the entoderm, we find that only a 

 few forms, with eggs comparatively poor in yolk, show a primitive 

 condition which can be compared directly to the method of develop- 

 ment among the Annelida. Here again Qetockilaa must be mentioned 

 first, as showing an archenteric vesicle arising by invagination which, 



