Rathke, Growth of the Eggs of the Crawfish. 245 
transparent watery fluid, which diminishes in quantity asthe embryo ad- 
vances in its growth, and at last vanishes altogether, the two coats coming 
into contact with each other; this the authour considers as Albumen. 
6. The outer coat, by means of which the egg is attached to the processes 
of the tail. This is scarcely half as thick as the coriaceous coat, but on 
account of the inequalities of its surface is much less transparent, and 
adheres to the coriaceous coat in every part, excepting only in that 
smooth portion of the latter, which has just been noticed ; in this place a 
minute cavity is formed between the two, which corresponds with the 
point of attachment between the ovum and the shell of its mother. 
The third section treats of the further developement of the embryo up 
to the period of the appearance of distinct organs, or the changes which 
occur in it during the month of April. The first change that takes place 
is the formation of a considerable number of insulated greyish white spots, 
of an irregularly roundish or elliptical form, over the whole surface of the 
vitellus. These patches, each of which is from four to six times as large 
as the largest globules of the vitellus, are connected together by minute 
filaments of the net work, of which the greater part of the embryo was 
previously composed. By degrees they assume a chalky whiteness, with 
a brown central point and a well marked circumference, gradually di- 
minishing in size and apparently also in number. After a time they 
again separate and the substance of the embryo is dispersed over the 
surface of the vitellus, forming where it is thickest a clouded appearance 
resembling a cirrus, and where it is thinner, appearing, under a strong - 
magnifying power, very finely reticulated, Every thing seems now 
prepared for the re-appearance of a proper embryonal sacculus. The 
scattered substance of the embryo contracts itself towards a certain point 
of the vitellus, leaving the far greater portion of the latter entirely free. 
This point is either actually beneath the attachment of the egg to the shell 
of its mother, or in the immediate neighbourhood, and never on the opposite 
side. As the embryo contracts itself, it mcreases in thickness in the 
middle, and becomes more definite at its edges. In this discoidal form 
its longest diameter is about half the radius of the egg, its colour is 
nearly uniformly white, and its constituent particles appear to be con- 
verted into granules, about equal in size to the largest globules of the 
vitellus, 
