430 



FORMATION OF THE LAYERS. 



circumference of the ovum in Oniscus (fig. 241 A) but is relatively 

 much shorter in Cymothoa. Anteriorly it dilates into the two pro- 

 cephalic lobes. In Cymothoa it next becomes segmented ; and the 

 anterior segments are formed nearly simultaneously, and those of the 

 abdomen somewhat later. At the same time a median depression 



FlG. 241. TWO LONGITUDINAL SECTIONS THEOUGH THE EMBRYO OF ONISCUS MURARIUS. 



(After Bobretzky.) 



st. stomodaaum ; pr. proctodseum ; hy. hypoblast formed of large nucleated cells 

 imbedded in the yolk ; ra. mesoblast; vg. ventral nerve cord; sg. supra-cesopbageal 

 ganglion; li. liver; do. dorsal organ; zp. rudiment of masticatory apparatus; ol. upper 

 lip. 



appears dividing the blastoderm longitudinally into two halves. The 

 appendages are formed later than their segments, and the whole of 

 them are formed nearly simultaneously, with the exception of the last 

 thoracic, which does not appear till comparatively late after the 

 hatching of the embryo. The late development of the seventh 

 thoracic segment and appendage is a feature common to the majority 

 of the Isopoda (Fritz Miiller). In Oniscus the limbs are formed in 

 nearly the same way as in Cymothoa, but in Asellus they do not arise 

 quite simultaneously. First of all, the two antennae and mandibles 

 (the future palp) appear, inaugurating a stage often spoken of as the 

 Nauplius stage, which is supposed to correspond with the free Nauplius 

 stage of Penaeus and Euphausia. At this stage a cuticle is shed (Van 

 Beneden) which remains as an envelope surrounding the larva till 

 the time of hatching. Similar cuticular envelopes are formed in 

 many Isopoda. Subsequently the appendages of the thorax appear, 

 and finally those of the abdomen. Later than the appendages there 

 arise behind the mouth two prominences which resemble appendages, 

 but give rise to a bilobed lower lip (Dohrn). 



In Asellus and Oniscus the ventral plate moulds itself to the shape 

 of the egg, and covers the greater part of the dorsal as well as of the 

 ventral side (fig. 241 A). As a result of this the ventral surface of the 

 embryo is throughout convex ; and in Asellus a deep fold appears on 

 the back of the embryo, so that the embryo -appears coiled up within 

 the egg with its ventral side outwards and its head and tail in contact. 

 In Oniscus the ventral surface is convex, but the dorsal surface is 

 never bent in as in Asellus. In Cymothoa the egg is very big and the 



