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 ege 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?. 
1 Erp did not perceive the cleaving in the eggs of Astacus marinus, but did in 
those gf*Cancer menas. In this last RATHKE also observed the phenomenon, as well 
as in Gammarus fluviatilis and G. Locusta, KOELLIKER in Hrgasilus and Cyclops. 
2 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 tiber 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 Bildungs-und Entwickelungsgesch. 1. 1833, 8. 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- 
phologie, Reisebemerkungen aus Taurien. Riga u. Leipzig, 1837, 4to. s. 35—151. See 
also the compressed review of this subject by RaTHKE, in the second edition of Bur- 
