358 



INVEKTEBEATA 



CHAP. 



constituting the rudiment of the hind foot or funnel. These are 

 termed the anterior and posterior funnel folds respectively. The 

 rudiments of the fore foot, the arms, appear as a series of thickenings 

 on the lower edge of the blastoderm (Figs. 288, 289). 



Whilst these changes have been going on, the first trace of the 

 mid-gut makes its appearance as a thickening of the " mesendoderm " 

 on the posterior slope of the egg ; whilst the stomodaeum appears as 

 an invagination in the middle line of the anterior slope of the embryo. 

 In the endodermal thickening a cavity is formed by the 

 separation of its constituent cells from each other. This cavity is 

 at first closed internally merely by the yolk -membrane ; but it soon 

 independently acquires its own internal wall, whilst externally it 



raises the ectoderm into a slight 

 papilla, the anal papilla, at the 

 apex of which the anus appears a 

 little later. There is no procto- 

 daeum. Before the anus is per- 

 forated the incipient mid-gut gives 

 off a diverticulum which is the rudi- 

 ment of the ink gland (Fig. 290, B). 

 The stomodaeum is at first a 

 shallow cup, but it soon extends up 

 towards the mid-dorsal line and past 

 it, where, in a much later perio^of 

 development, it fuses with and^ens 

 into the mid-gut. The radula sac 

 appears as a ventral outgrowth of 

 the stomodaeum, and a still more 

 ventral outgrowth is the rudiment 

 of the salivary gland. The oto- 

 cysts now arise as open pits near 

 the junction of the anterior and 

 posterior funnel folds. These 

 two pairs of folds, as already noted, 

 are the rudiments of the funnel. 

 In this stage the rudiments of the three principal pairs of 

 ganglia arise. The cerebral ganglia appear as long streak-like 

 ectodermic thickenings running below the eyes ; the pedal ganglia 

 as much shorter streaks parallel to the hindermost portions of these 

 and above them, extending to the otocysts ; and finally the pleural 

 ganglia are represented by short vertical streaks extending up 

 towards the mantle. The shape of the rudiments reminds one of the 

 band-Hke condition of the ganglia found in Nautilus (Fig. 291), 

 and we may add that the appearance of the funnel as two un- 

 connected ridges also recalls its condition in Nautilus. 



The first trace of the genital cells appears in this stage as a 

 packet of large pale cells with pale nuclei, situated on the posterior 

 aspect of the embryo between the rudiment of the gills (Fig. 297, A). 



Fig. 289. — Embryo of Loligo vulgaris 

 seen from the posterior side at the 

 conclusion of the period of develop- 

 ment, termed by Faussek Stage 1. 

 (After Faussek.) 

 a, anus ; a.f.f, anterior funnel fold ; ar, 

 rudiment of arras ; br, gill rudiment ; m./, 

 mantle fold ; j)././, posterior funnel fold ; o.st, 

 eye-stalk ; ot, otocyst ; s.s, shell-sac. 



