612 CLASS X. 



from a coagulable fluid, which has been secreted by the oviduct. 

 This covering is thicker than the vitelline membrane, and between 

 the two only a small space or none at all remains ; in the first case 

 a watery albuminous fluid occupies it. The production of the germ 

 is, as in the egg of many other animals, both vertebrate and inver- 

 tebrate, so also in that of many crustaceans, preceded by a parting 

 and cleaving of the yolk *. There arise first in the yolk a greater 

 or lesser number of membranous saccules, follicles which probably 

 take their origin from the albuminous fluid portion of the yolk, 

 and enclose a greater or lesser number of yolk-cells. The germ 

 first appears as a nebulous grey spot, which consists of cells that 

 have nuclei, and probably arise from modified yolk-cells. At first 

 the germ is small, afterwards the germinal membrane grows round 

 the whole of the yolk. It separates into two layers, which may 

 be compared to the serous lamina and the mucous lamina in the 

 blastoderma of vertebrates. Just as in insects and arachnids, the 

 ventral parts of the walls of the body are the first that come into 

 being. The parting of the body into rings or segments begins on 

 the ventral surface. All the appendages (limbs, jaws, feet) greatly 

 resemble one another at first, and on the whole the anterior mem- 

 bers are formed first, the posterior last. Many crustaceans make 

 their appearance from the egg with fewer limbs than they afterwards 

 possess. But the development of crustaceans presents many dif- 

 ferences, in the different orders, of which the particular description 

 would demand too large a space. Such an uniformity of the plan 

 of development, as we observe in the classes of vertebrate animals, 

 seems in the lower classes of animals not to occur 2 . 



1 ERDL did not perceive the cleaving in the eggs of Astacus marinus, but did in 

 those of Cancer mcenas. In this last EATHKE also observed the phenomenon, as well 

 as in Gammarus fluviatilis and 0. Locusta, KOELLIKER in Ergasilus and Cyclops. 



3 As the comparative history of development in general, so especially has that of 

 crustaceans, received its clearest illustration from the unwearied and distinguished 

 investigations of H. RATHKE. We cite here his Untersuchungen uber die Bildung u. 

 Entwickelung des Flusskrebses, mit 5 Kupfert. Leipzig. 1829, folio (from which ample 

 extracts are given with many figures in the Ann. des Sc. natur. Tom. xx. 1830, 

 pp. 442 469); Abhandlungen zur BUdungs-und EntwicTcelungsgescJi. n. 1833, s. 69 

 94, (in Asellus aquaticus, Oniscus murarius, Daphnia, Lynceus), and especially for the 

 numerous investigations in very different families of crustaceans, his work, Zur Mor- 

 phologic, ReisebemcrTcungen aus Taurien. Riga u. Leipzig, 1837, 4^- s - 35 I 5 T - ^ ee 

 also the compressed review of this subject by RATHKE, in the second edition of BUB- 



