336 LECTUBE XV. 



appearance of the elaborate monograph by Rathke, on the develop- 

 ment of a freshwater species. 



In the year 1829, Dr. Rathke published his " Researches on the 

 River Craw-fish (Astacus Jluviatilis).* In this species the ovum 

 first appears in the shape of a minute transparent vesicle, which 

 afterwards becomes surrounded by a second, forming the membrana 

 vitelli : the nature of the processes effecting this stage appears not to 

 have been observed. The yolk increases in quantity, and is rendered 

 opaque by the presence of numerous granules, or nucleated cells, 

 which are at first angular, and change from the lenticular to 

 the spherical figure ; then the internal minute transparent germinal 

 vesicle quits the centre, and comes into contact with one part of the 

 parietes of the ovum. The colour of the yolk successively changes 

 to yellow, orange, and brown ; the spermatozoa, enveloped in their 

 spermatophore, when they are received into the oviduct, escape 

 therefrom, and ascend the duct to the ovarium, and after coming into 

 contact with the ovum, the clear vesicle disappears, and the produc- 

 tion of the embryo commences. Rathke failed to ascertain what 

 became of the vesicle. The formation of the ovum in the ovary 

 continues half a year. In the month of November, the vesicle was 

 visible ; in the ensuing March it had disappeared. 



The ovum escapes into the oviduct by bursting the inner lining of 

 the ovary. It is surrounded by a layer of albuminous matter, and is 

 enclosed within a coriaceous chorion, and an irregularly doposited 

 nidamental tunic, by which the ovum, after exclusion, becomes 

 attached to the ciliated plates beneath the tail of the mother. 



The first appearance of development is as a whitish cloud of 

 indeterminate form, spreading over the vitellus, and assuming, as it 

 extends, a reticulated appearance. It seems as if the germ-cells had 

 propagated themselves over the superficies of the yolk-mass. A 

 discoid portion of the opake layer is defined from the rest, and 

 increases in thickness at its middle part: its longest diameter is 



136^--^ -^ about half the radius of the egg. A depression 



appears in the centre of this, whicli passes more 

 and more deeply into the vitellus, and the em- 

 bryonic spot expands at its m.argins The patch 

 next grows heart-shaped, and the antenna3. the 

 labrum, mandibles, and abdomen become simul- 

 taneously recognizable {Jig. 136). The first 

 „ , .,, appearance is a minute median prominence, 



Astaciis fluviatllis. .1 /. 



which becomes the labrum e ; that part of the 

 mouth which we find most constantly in the Crustacea. Then the 



* CCXXXVII 



