History of a transient nervous apparatus in certain Ichtbyopsida. 349 



remain in contact with those actually on the cord, and on the other 

 pass into long nerve-processes (s. n), the products of their own spinning 

 activities. The two fibres could be followed just as far as, and no 

 further than, represented in the figure. The one on the left comes 

 to an abrupt end nearly opposite the segmental duct, whereas the other 

 could be traced for some little distance further ventralwards. 



2) Embryos of 12—13 mm. 



In the first embryo (No. 180) to be described under this heading 

 the characters of the transient nervous apparatus resemble in many 

 respects those of No. 141, but the system, so far as its ganglion- 

 cells are concerned, has made great progress in its development. 

 Some of its conditions are so striking and remarkable that, were it 

 not for the constant recurrence of similar appearances in a large 

 series of embryos, one might be inclined to suspect some abnormality. 



R. hatis No. 180 measured 12 mm and possessed about 111 somites 

 behind the last gill- cleft. Of the gill-pouches all six are present, 

 all but the last have openings to the exterior, and there are external 

 gill-buds on two arches. 



The neurenteric canal is still persistent — it does not disappear 

 until the last somite has been formed. 



In the eye the front wall of the retina is thickened and invaginated 

 into the posterior wall. The lens has the form of an epiblastic thicken- 

 ing projecting into the optic cup. 



The paired fins are appearing as bud-like outgrowths of the body 

 wall. The segmental duct reaches as far as the cloaca, but does not 

 open into it, being fused with the epiblast in front of the cloaca. 

 Like No. 141 the series contains abundant "primitive ova" in the 

 region where the sexual organs will arise, but here there are none in 

 the somatopleure, which is actively proliferating formative tissue into 

 the limb-outgrowths and for the cutis. 



Several sections from the series are figured on Plate 24. (Figs. 51, 

 52, 53, 54 and 55). 



Fig. 51, one of the most anterior, is also one of the most note- 

 worthy of these figures. As the drawing shows, there are within or 

 upon the cord (in which the "Randschleier" of His has begun to be 

 formed), five large ganglion-cells in the usual position. On the left 

 side, and out from the cord, there project five other similar ganglion- 

 cells (w.gl.c) forming a chain, the one end of which is applied to a 

 short stumpy process of the outermost of the five central cells, while 



