152 EMBRYOLOGY CHAP. 



ventral parts of the embryo are well advanced, while the dorsal 

 part of the embryo is still quite incomplete. 



The method of investigation by which embryologists chiefly 

 carry on their researches is that of dividing the egg after 

 proper preparation, into a large number of thin sections, which 

 are afterwards examined in detail, so as to allow the arrange- 

 ment to be completely inferred and described. Valuable as this 

 method is, it is nevertheless clear that it should, if possible, 

 be supplemented by direct observation of the processes as they 

 take place in the living egg : this method was formerly used, 

 and by its aid we may still hope to obtain exact knowledge 

 as to the arrangements and rearrangements of particles by 

 which the structures develop. Such questions as whether the 

 whole formative power in the egg is absolutely confined to 

 one or two small centres to which the whole of the other egg 

 contents are merely, as it were, passive accessories, or whether an 

 egg is a combination in which some portion of the powers of 

 rearrangement is possessed by other particles, as well as the 

 chromosomes, in virtue of their own nature or of their position 

 at an early period in the whole, can scarcely be settled without 

 the aid of direct observation of the processes during life. 



The importance of the yolk is recognised by most of the 

 recent writers. Nussbaum states (loc. cit.') that " scattered yolk- 

 cells associate themselves with the mesoblast cells, so that the 

 constituents of the mesoblast have a twofold origin." Wheeler 

 finds * that amoeboid cells he styles them vitellophags 

 traverse the yolk and assist in its rearrangement ; he insists on 

 the importance both as regards quantity and quality of the yolk. 



The eggs of some insects are fairly transparent, and the 

 process of development in them can, to a certain extent, be 

 observed by simple inspection with the microscope ; a method 

 that was used by "Weismann in his observations on the embry- 

 ology of Chironomus. There is a moth (Limacodes testudo\ that 

 has no objection to depositing its eggs on glass microscope-slides. 

 These eggs are about a millimetre long, somewhat more than half 

 that width, are very flat, and the egg-shell or chorion is very thin 

 and perfectly transparent. When first laid the contents of this 

 egg appear nearly homogeneous and evenly distributed, a finely 

 granular appearance being presented throughout ; but in twenty- 

 1 /. Morphol. viii. 1893, pp. 64, 65, and 81. 



