4 ARACHNIDA. 



(Fig. 3, A and B). At first this is an irregular mass, extending over 

 the whole region of the germ-disc, but at a later stage it takes the 

 form of two symmetrically arranged bands situated near the middle 

 line. These two bands, which fuse with one another posteriorly,, 

 become divided up later into the primitive mesodermal segments, 

 each of which contains a cavity (Fig. 3, B, mes). 



The increase in amount of the mesoderm is due to a multiplication 

 of the cells of the primitive entoderm from which, in places, it is not 

 yet differentiated (Laurie). If it were the case that the principal 

 increase of the cells proceeded from a point at the posterior end of 

 the germ-disc, a growth of the mesoderm-bands from behind forward 

 would result, such as is found in many other segmented animals. 

 The differentiation of the mesoderm-bands would then proceed here, 

 as in those forms, from before backward. 



2. The Origin of the Embryonic Membranes and the Development 

 of the External Form of the Body. 



During the processes just described, the germ-disc has extended 

 but little over the yolk, and still appears as a rounded, or somewhat 

 oval, disc. Even at this early stage the formation of embryonic 

 membranes begins. A groove appears near the periphery of the 

 germ-disc, running right round it and marking off the central 

 portion of the disc in the form of a slight prominence rising from 

 the narrow peripheral area. At the edge of the groove a fold of the 

 ectoderm rises, and this now grows from the periphery over the germ- 

 disc, finally fusing at its centre. The two lamellae of the embryonic 

 envelope are thus formed. The outer membrane, which lies imme- 

 diately below the egg-integument, is the serosa, and the inner, the 

 amnion. During the formation of this ectodermal fold a few meso- 

 derm cells are said to pass in between its two lamellae (Kowalevsky 

 and Schulgin). 



According to Kowalevsky and Schulgin, the formation of the embryonic 

 envelopes in the Scorpiones (at least in Androdonus) takes place in the same way 

 as in the Insecta and Vertebrata. Lauiiie, who investigated Euscorpius italicus, 

 came to a different conclusion regarding the origin of these membranes, he thought 

 that the two cell-layers which form the serosa and the amnion grow indepen- 

 dently over the germ-disc from its periphery. The serosa appears first as a 

 layer of cells which rises all round the edge of the germ-disc, and grows towards 

 the centre, where it fuses. Only after this fusion does a second layer, the 

 amnion, appear, and pass through the same process (Fig. 3, A and B). Such 

 an outgrowth of a single layer of cells is difficult to understand, and must 

 no doubt have originated as a fold of ectoderm consisting of two layers of cells. 



