THE DEVELOPMENT OF THE SQUID. 



36^ 



of the mouth there is a very iaintly-marked undulating 

 line of cilia, the velum (v). 



Three pairs of rudimentary arms («) 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 sh;ipe 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. 103, repre- 

 sented with its dorsal sui-face above, and 

 showing the anterior surface of the 

 body, as seen from the left side. ( Drawn 

 from natnre by W. K. Brooks. ) 



The letters of reference have the 

 following significance in all the figiu'es 

 which follow : — 



a. Arms. a'. Posterior or siphonal 

 pair of arms. a". Second pair. 

 a". Third pair. e. Eyes. er. Oto- 

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

 h. Branchial hearts. jn. Mantle. 

 mo. Mouth, re. Rectum, si. Inner 

 siphon-folds, si'. Lateral siphon-folds. 

 sm. Siphonal muscle. r. Velum. 

 ?/. Yolk. ]/'. External yolk-sac. y''. Xolk mass of the eye-stalks and 

 head, y" and y' ". 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 cavitv which is larsrest alonor the 

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

 few branched corpuscles, by the contraction of which. 



