^2 PROCEEDINGS OF THE ACADEMY OF [Feb., 



iliual grooves ; neither did the duct leading to the pharynx, so these 

 features were omitted in this illustration. The longitudinal grooves 

 are exaggerated somewhat in the drawing. The gland, in life, had 

 a faintly yellow color, while the rest of the larvse, except the 

 blood, was nearly white, with numerous many-branched, black 

 pigment spots resembling very complicated nerve cells. The living 

 larva of this age is quite transparent, so that the pulsation of the 

 heart and of the velum (v. ) and the circulation of the blood 

 through the branchial arches can be seen easily under a low power. 

 The thyroid, being rather more opaque than the rest of the animal, 

 shows very plainly by contrast. In the figure the outlines of the 

 pharynx and the positions of the arches are shown by the broad 

 dotted lines. The characteristic hood-like projection over the 

 mouth is also shown. With the exception of the eye (e), no 

 attempt has been made to represent any of the other organs of the 

 larva. 



A ventral view of the larva of this age shows the thyroid as a 

 clearly defined elongated body, lying against the ventral wall of 

 the pharynx. 



By this time great advances have been made in the development 

 of the thyroid and of the ciliated grooves. Fig. 56 is a transverse 

 .section a short distance anterior to the thyroid. It shows the spinal 

 cord (.s/>.c. ), notochord (c^. ), body wall (b.tv.), etc., in outline, 

 while the cell structure of the pharynx (phar. ) is shown. This 

 section, on the right side, passes through the ciliated groove at the 

 point (I) where its anteriorly directed ventral branch (v. e.g., on 

 the left) turns dorsalward to run posteriorly as the dorsal groove 

 (d.c.g., on the left). On the left of the section the two branches 

 of the groove are distinct as a dorsal {d.c.g.) and ventral (v. e.g.) 

 groove. These grooves are lined with cells which are much higher 

 and more columnar than the cells of the rest of the pharyngeal 

 wall, so that they are easily followed, by sections, throughout their 

 course. Their cilia are short and indistinct. 



The mesoblast (mes. ) surrounding the thyroid and pharynx is 

 now quite different from that of earlier stages, being composed of 

 .scattered angular cells instead of closely packed, more or less oval 

 cells. The yolk has disappeared. The outline of the pharynx is 

 represented in a somewhat diagrammatic manner, though it was 

 traced as accurately as possible under a camera. 



