150 



CRUSTACEA. 



the egg. Larval integuments like that in the Nauplius of Asellus are of very 

 common occurrence among the Arthrostraca. 



An apparent exception to the rule that the germ-hand in the Isopoda exhibits 

 a dorsal curvature in its early stages is afforded by Cymothoa (Bullae, !N"o. 81). 

 The germ-band, which here lies on the ventral side of the egg, whic.li is very 

 large and richly supplied with food-yolk, shows for the greater part of its 

 course the same dorsal curvature as that of other Isopoda. Its most posterior 

 end (the rudiment of the telson) is, however, bent round ventrally. 



As the germ-band broadens in later stages, its lateral portions grow 

 up over the dorsally superimposed food-yolk, and thus give rise to 

 the lateral parts of the embryo. The further extension of this 

 growth finally leads to the coalescence of the lateral edges of the 

 germ-band in the dorsal middle line, whereby the enclosure of the 

 mass of yolk within the embryo is completed. It should be noted 

 that during this process that part of the ectoderm which formerly 



covered the dorsal accumu- 

 lation of yolk is crowded 

 into a smaller space, and 

 finally degenerates. This 

 process has apparently some 

 connection with the de- 

 velopment in the dorsal 

 region of many Arthro- 

 stracan embryos of an organ 

 discovered by Mbissnbr in 

 Gammarus and described 

 as a mlcropyle apparatus; 

 this was found later in 

 many Amphipoda and a 

 few Isopoda, and, as the 

 spherical or dorsal organ, 

 it has received many differ- 

 ent explanations. In the Amphipoda, the dorsal organ appears 

 during the differentiation of the germ-band as a dorsal disc-shaped 

 thickening of the blastoderm (Fig. 75, K) projecting inwards towards 

 the yolk ; when the larval integument develops, this organ adheres 

 closely to it. A perforation is formed here (the micropyle), while 

 the central parts of the disc form a small cavity by invagination 

 (Fig. 76). In later ontogenetic stages, when the heart develops 

 below the dorsal organ, the latter degenerates, mesoderm-cells 

 wandering in between its cells and apparently taking an active 

 part in its disintegration (Rossijskaya, No. 72). 



Fig. 76. — Diagrammatic transverse section through 

 the embryo of Cymothoa (after Bullar, from 

 Balfour's Text-book). Below is seen the germ- 

 band in cross section, above, the dorsal organ 



