290 THE ANATOMY OF LNVERTEBRATED ANIMALS. 



motionless, and have the form of cells, provided with a nu- 

 cleus and pr6duced into several delicate radiating processes. 

 They are united in their course down the vas deferens into 

 cylindrical masses, which, becoming invested by a fine mem- 

 branous coat, probably secreted by the walls of that duct, 

 constitute the spermatophores, which may not unfrequently 

 be found adhering to different parts of the body, not only of 

 female but of male Crayfish. 



The ova are fecundated while still within the parent ; they 

 become surrounded, in their passage down the oviduct, by a 

 coat corresponding with that of the spermatophore, which is 

 produced into a pedicle, the extremity of which becomes at- 

 tached to one or other of the abdominal appendages. Great 

 numbers of ova, attached in this way, may be observed, dur- 

 ing the breeding-season, within the incubatory chamber formed 

 by the flexure of the abdomen upon itself; and it is in this 

 cavity that the embryos pass through the whole of their fcetal 

 existence. 



The development of the Crayfish has been the subject of 

 one of the most beautiful of the many admirable memoirs on 

 development, for which we are indebted to the genius and 

 patience of Rathke. 1 After fecundation a blastoderm arises 

 upon the surface of the yelk, and, gradually extending over 

 the whole yelk, becomes thickened at one part, so as to form 

 an oval germinal disk, with a central depression. 



This disk next becomes widened and bilobed at its ante- 

 rior extremity, the lobes being identical with the procephalic 

 lobes, to be hereafter described in the embryo of Mysis. The* 

 edges of the disk are raised into a fold, and within the fold a 

 papilla, the rudiment of the abdomen, and of the greater part 

 if not of the whole of the thorax, makes its appearance ; while, 

 anteriorly, three pairs of transverse elevations constitute the 

 rudiments of the antennules, the antennae, and the mandibles. 

 The labrum arises as a median papilla, situated at first be- 

 tween the antennules. The ocular peduncles are next devel- 

 oped in front of the antennules as ridges, which only subse- 

 quently become free processes. 



The thoracico-abdominal process lengthens, and the anal 

 aperture makes its appearance. It is to be remarked that 

 the anus is at first situated on the dorsal side of the extremity 



1 "Ueber die Bildung und Entwickelung des Flusskrebses," Bd. 29. See 

 also Lereboullet, " Recherches d'Embryologie comparee sur le t)eveloppement 

 du Brocbet de la Perche et de 1'Ecrevisse/' 1862 ; and the account of Bobret- 

 sky's researches in Hofmann and Schwalbe, " Jahresbericht " for 1873 (1875). 



