98 F. M. BALFOUR. 



out it is formed from a solid cord of the cells of the dorsal 

 region. The peripheral layer of this cord gives rise to the 

 walls of the heart, while the central cells become converted 

 into the corpuscles of the blood. 



The rudiment of the heart is in contact with the epiblast 

 above, and there is no greater evidence of its being derived 

 from the splanchnic than from the somatic mesoblast ; it is, 

 in fact, formed before the dorsal mesoblast has become dif- 

 ferentiated into two layers. 



In the abdomen three or four transverse septa, derived 

 from the splanchnic mesoblast, grow a short way into the 

 yolk. They become more conspicuous during the succeed- 

 ing stage, and are spoken of in detail in the description of 

 that stage. In the anterior part of the thorax a longitu- 

 dinal and vertical septum is formed, which grows downwards 

 from the median dorsal line, and divides the yolk in this 

 region into two parts. In this septum there is formed at a 

 later stage a vertical muscle attached to the suctorial part 

 of the stomodseum. 



The mesoblastic somites of the earlier stage are but little 

 modified ; and there are still prolongations of the body 

 cavity into the limbs (PL X, fig. 18). 



The lateral parts of the ventral nerve cords are now at 

 their maximum of separation (PI, X, fig. 18, v, g). 

 Considerable diff"erentiation has already set in in the con- 

 stitution of the ganglia themselves, which are composed of 

 an outer mass of ganglion cells enclosing a kernel of nerve 

 fibres, which lie on the inner side and connect the successive 

 ganglia. There are still distinct thoracic and abdominal 

 ganglia for each segment, and there is also a pair of separate 

 ganglion for the chelicerse, which assists, however, in forming 

 the CESophageal commissures. 



The thickenings of the prseoral lobe which form the supra- 

 oesophageal ganglia are nearly though not quite separated from 

 the epiblast. The semicircular grooves of the earlier stages 

 are now deeper than before, and are well shown in sections 

 nearly parallel to the outer anterior surface of the ganglion 

 (PI. X, fig. 19). The supra-cesophageal ganglia are still 

 entirely formed of undifferentiated cells, and are without 

 commissural tissue like that present in the ventral ganglia. 



The stomodseum has considerably increased in length, and 

 the proctodaeum has become formed as a short, posteriorly 

 directed involution of the epiblast. I have seen traces of 

 what 1 believe to be two outgrowths from it, which form 

 the Malpighian bodies. 



The next stage constitutes (PI. VIII, fig. 9) the last which 



