THE DEVELOPMENT OF THE SQUID. 



369 



of the mouth there is a very faintly-marked undulating 

 line of cilia, the velum (v). 



Three pairs of rudimentary arms (a) are now present 

 upon the sides of the body, about half- way between the 

 mantle and the opposite pole of the egg. The yolk is now 

 entirely surrounded by the blastoderm, and has departed 

 still further from the regularly curved shape of Fig. 191. 

 The prolongations into the mantle and the eye-stalks are 

 well defined, and the portion of the yolk contained within 



FIG. 195. Embryo a little older 

 than the one shown in Fig. 19:], repre- 

 sented with its dorsal surface above, and 

 showing the anterior surface of the 

 body, as seen from the leftside. (Drawn 

 from nature by W. K.. Brooks. ) 



The letters of reference have the 

 following significance in all the figures 

 which follow: 



". Arms. a. Posterior or siphonal 

 pair of arms. a". Second pair, 

 a". Third pair. c. Eyes. er, Oto- 

 cyst. es. Eye-stalk. /. Fins. g. Gills. 

 h. Branchial hearts. m. Mantle. 

 mo. Mouth, re. Rectum, si. Inner 

 siphon-folds. */'. Lateral siphon-folds. 

 KIH. Siphonal muscle. v. Velum. 

 ;//. Yolk. //'. External yolk-sac. ?/''. Yolk mass of the eye-stalks and 

 head. ?/"' and ?/' ". Yolk masses of the body and mantle. 



the body of the embryo, which is not quite half the whole, 

 is separated by a well-marked constriction, just dorsal to 

 the arms from the remainder, which is now nearly spheri- 

 cal. The thin layer of blastoderm which covers this ex- 

 ternal portion of the yolk is split into two layers, separated 

 from each other by a cavity which is largest along the 

 median plane of the body, and which is traversed by a 

 few branched corpuscles, by the contraction of which, 



FIG. 195. 



