364 



INVERTEBRATA 



CHAP. 



~fn 



'.S 



ventral diverticula grow out, which are the rudiments of the salivary 

 glands. The mid -gut becomes differentiated into a stomach 

 and an intestine, and the diverticulum which forms the ink sac 

 opens into the latter. The liver arises as two lateral outgrowths 

 of the stomach. The surfaces of these outgrowths become folded, 

 and this is the first indication of the formation of the liver tubules. 

 The coelom, which is already constricted into kidney and pericar- 

 dium, now increases greatly in volume. The two pericardial rudiments 



fuse behind and enclose the two 

 rudiments of the heart ; these 

 latter likewise fuse together. 

 In front the pericardial rudi- 

 ments remain separate and are 

 applied to the paired rudiments 

 of the heart which here con- 

 stitute the auricles. The paired 

 portions of the pericardium 

 communicate with the kidneys 

 and give rise to the reno- 

 pericardial canals. Behind, 

 the single pericardium grows 

 backward and extends into 

 the growing genital organ, 

 which becomes divided up into 

 the genital folds. This portion 

 of the pericardium becomes, 

 later, divided off from that sur- 

 rounding the heart and forms 

 the genital coelom (Fig. 297). 



The kidney sacs develop a 

 high columnar epithelium on 

 their inner walls,| where they 

 are in contact with the forks 

 of the vena cava ; the epithelium 

 lining their outer walls becomes 

 very thin. The posterior sinus 

 is reduced to very small 

 dimensions by the expansion 

 of the shell-sac, and is cut off from the vena cava. The portions 

 of the vena cava which extend into the gills and constitute 

 the branchial hearts, develop great thickenings of their walls on 

 one side. These thickenings, as experiment has proved, are excretory 

 in nature and consist of vacuolated cells ; they are covered 

 externally by thin peritoneal epithelium where they touch the 

 coelom. These are of course the appendages of the branchial heart. 

 The cartilage so characteristic of Cephalopoda is formed by the 

 modification of mesodermic connective tissue, and is first visible 

 in the neighbourhood of the foot. The chromatophores likewise, 



FIG. 296. Embryo of Loligo vulgar is at the 

 period when the funnel is formed, viewed 

 from behind. (After Korschelt.) 



ar, arms ; cn.f, corne il fold which envelops the 

 eye-stalk, and forms the outer chamber of the eye ; 

 /, completed funnel ; fn, (ins ; in,./, mantle edge ; oc, 

 eye ; y.s, yolk-sac. 



