138 



CRUSTACEA. 



recently by Beugh (Vol. i., p. 294, note). The symmetrical growth 

 of the mesoderm-bands appears, however, to prove that in Ligia and 

 Oymothoa, the two original formative zones of the mesoderm 

 correspond in position to the posterior end of the future germ-band. 

 We may therefore suggest that this is the case in Mysis also, and 

 that the two lateral lobes of the germ-disc above-mentioned answer 

 to the paired zone of formation of the mesoderm. In this case we 

 might assume that the entire mesoderm of the germ-band arises bv 

 proliferation which starts from this formative zone, and not, as 

 Nusbaum assumes for Mysis, by a kind of delamiiiation from the 

 inner surface of the ectoderm of the germ-band. 



A 



Fig. 69.— Germ-band in the Nau- 

 plius stage of Ligia oeeanica 

 (after Nusbaum) a, anus ; h, 

 entoderm-lobes (hepatic rudi- 

 ments) ; o, optic-lobes ; 1, 2, 3, 

 first, second, and third pair of 

 Nauplius limbs ; ol, upper lip ; 

 k, formative zone of the seg- 

 ments. 



Fig. 70.— A portion of the segment-forming zone of the 

 germ-band in Cymothoa (after Patten). A, median 

 longitudinal section. B, inner aspect of the posterior 

 end of the germ-band (Fig 69). On the right the three 

 lateral cells of the mesoderm-layer are omitted, ec, 

 ectoderm ; ms, mesoderm. 



In Lvjia and in the Isopoda generally, there is only a small 

 number of yolk-cells, which play the same part as those of Mysis. 

 These separate in early stages from the lower layer of the embryonic 

 rudiment, wander into the yolk, as vitellophags take part in its 

 disintegration, and finally disappear. The actual entoderm does 

 not here pass into the yolk, but forms a paired mass of cells, which 

 remains in close connection with the germ-band and soon becomes 

 arranged in two halves (Fig. 69, h). As to the way in which the 

 future mid-gut is formed out of these rudiments, Nusbaum's account 



