Rathke, Growth of the Eggs of the Crawfish. 245 



transparent watery fluid, which diminishes in quantity as the 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. 



