NOTES ON THE DEVELOPMENT OF THE ARANETNA. 685 



The peripheral layer of this cord gives rise to the walls of the 

 heart, while the central cells become converted into the cor- 

 puscles 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 differentiated 

 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 succeeding stage, 

 and are spoken of in detail in the description of that stage. 

 In the anterior part of the thorax a longitudinal 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 32, fig. 18). 



The lateral parts of the ventral nerve cords are now at their 

 maximum of separation (PI. 32, fig. 18, v. -.). Considerable 

 differentiation has already set in in the constitution 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 chelicerae, 

 which assists, however, in forming the cesophageal commissures. 

 The thickenings of the praeoral lobe which form the supra- 

 cesophageal 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 shewn in sections nearly 

 parallel to the outer anterior surface of the ganglion (PI. 32, 

 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 stomodsum has considerably increased in length, and 

 the proctodseum has become formed as a short, posteriorly 



