THE MESODERMAL STRUCTURES. 



*r> 



Should it be proved that the Malpighian vessels belong to the 

 enteron, we should have another argument against a close relationship 

 between the Arachnida on the one side, and the Myriopoda and 

 Insecta on the other. The Malpighian vessels must then be regarded 

 as resembling the appendages of the enteron in some Crustaceans, 

 and could no longer be compared with the synonymous vessels in the 

 Insecta. 



The Mesodermal Structures. 



The still unsegmented germ-band (Fig. 23 C, p. 46) becomes 

 distinguished from the rest of the blastoderm partly by the cylin- 

 drical nature of 

 the ectoderm-cells, 

 and by the growth 

 of the subjacent 

 mesoderm - layer. 

 This latter is at 

 first a continuous 

 band, which, as a 

 single layer, occu- 

 pies the whole area 

 of the germ-band 

 (Fig. 43 B). It, 

 however, soon 

 becomes multi- 

 laminar by active 

 increase of its 

 cells, and now 

 undergoes differ- 

 entiation into two 

 mesoderm - bands 

 divided by a slit, 

 which appears along the middle line (Balfour, Locy). This 

 occurs at a time when the germ-band externally shows division 

 into six segments (Fig. 25 A, p. 48, and Fig. 43 A and C). The 

 external segmentation seems to precede internal segmentation : this 

 latter, however, soon takes place, the mesoderm-bands breaking up 

 into the primitive segments in which the segmental cavities appear 

 (Fig. 43 A and C). Spaces, entirely free from mesoderm, occur 

 between the consecutive primitive segments (Schimkewitsch, Morin, 

 Fig. 43 .4). In the cephalic region, and especially in the caudal 

 region, where the differentiation of the mesoderm into primitive 





Fig. 44. — Longitudinal section through an embryo of Agalena 

 labyrinthica, somewhat at the same stage as in Fig. 27 (after 

 Balfour). The section is taken slightly to one side of the 

 middle line to show the extension of the primitive segments 

 into the limbs. In the centre is the yolk with the yolk-cells. 

 do, the small portion of the yolk not covered by the germ- 

 band ; pr.l, cephalic lobe ; 1-16, the body-segments ; 1, cheli- 

 cerae ; ~, pedipalps ; 3, first pair of legs, etc. 



