NOTES ON THE DEVELOPMENT OF THE ARANEINA. 683 



cephalic lobes. Such an elongation necessarily separates the 

 anal and procephalic lobes ; but, since the ventral plate does 

 not become shortened in the process, and the embryo cannot 

 straighten itself on account of the egg-shell, it necessarily be- 

 comes flexed, and such flexure can only be what I have already 

 called a ventral flexure. If there were but little food yolk this 

 flexure would cause the whole embryo to be bent in, so as to 

 have the ventral surface concave, but instead of this the flexure 

 is confined at first to the two bands which form the ventral 

 plate. These bands are bent in the natural way (PI. 30, fig. 8, B', 

 but the yolk forms a projection, a kind of yolk-sack as Barrois 

 calls it, distending the thin integument between the two ventral 

 bands. This yolk-sack is shewn in surface view in PI. 30, fig. 8, 

 and in section in PI. 32, fig. 18. At a later period, when the 

 yolk has become largely absorbed in the formation of various 

 organs, the true nature of the ventral flexure becomes apparent, 

 and the abdomen of the young Spider is found to be bent over 

 so as to press against the ventral surface of the thorax (PI. 30, 

 fig. 9). This flexure is shewn in section in PI. 32, fig. 21. 



At the earliest stage of this period of which I have ex- 

 amples, the dorsal region has somewhat increased, though not 

 very much. The limbs have grown very considerably and now 

 cross in the middle line. 



The ventral ganglia, though not the supra-cesophageal, have 

 become separated from the epiblast. 



The yolk nuclei, each surrounded by protoplasm as before, 

 are much more numerous. 



In other respects there are no great changes in the internal 

 features. 



In my next stage, represented in PI. 30, figs. 8 a, and 8 b, a 

 very considerable advance has become effected. In the first 

 place the dorsal surface has increased in length to rather more 

 than one half the circumference of the ovum. The dorsal region 

 has, however, not only increased in length, but also in definite- 

 ness, and a series of transverse markings (figs. 8 a and b}, which 

 are very conspicuous in the case of the four anterior abdominal 

 segments (the segments with rudimentary appendages), have 

 appeared, indicating the limits of segments dorsally. The terga 

 of the somites may, in fact, be said to have become formed. 



442 



