96 F. M. BALFOUR. 



anal segment will come to ocrupy a position on the ventral 

 surface, and the germinal bands to approach, but in the 

 inverse way, so as to form an angle opposite to that which 

 they formed at first. This condition ends the process by 

 ■which the posterior extremity of the embryonic band, at first 

 directed towards the dorsal side, comes to bend in towards 

 the ventral region." 



Neither of the above explanations is to my mind perfectly 

 satisfactory. The whole phenomenon appears to me to be 

 very simple, and to be caused by the elongation of the 

 dorsal region, i. e. the region on the dorsal surface between 

 the anal and procephalic lobes. Such an elongation neces- 

 sarily 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 

 eorcr-shell, it necessarily becomes flexed, and such flexure can 

 only be what 1 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 con- 

 cave, but instead of this the flexure is confined at first to the 

 two bauds which form the ventral plate. These bands are 

 bent in the natural way (PL YIII, fig. 8, b), but the yolk 

 forms a projection, a kind of yolk sack as Barrois calls it, dis- 

 tending the thin integument between the two ventral bands. 

 This yolk sack is shown in surface view in Pi. YIII, fig. 8, 

 and in section in PL X, 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 fPl. YIII, fig. 9). This flexure is 

 shown in section in PL X, 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 

 9101C cross in the middle line. 



The ventral ganglia, though not the supra-oesophageal, 

 have become separated from the epiblast. 



The volk 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 PL YIII, figs. 8 a, and 

 8 &, a very considerable advance has become eflfected. In 

 the first place the dorsal surface has increased in length to 

 rather more than one half the circumference of the ovum. 



