12 HENRY MCELDERRY KNOWER 



side to side, especially dorsally above the tubular yolk mass. 

 Since the inner parts still lie at the surface, unobscured by 

 connective tissue or other later overgrowths, the regional 

 features and primary subdivisions of organs are exceptionally 

 distinct. This is accentuated in fixed specimens, which are 

 particularly favorable for study when cleared in cedar-wood 

 oil, with oil of wintergreen (methyl salicylate) added or en- 

 tirely substituted, according to the increased degree of trans- 

 lucence desired. 



ARRANGEMENT OF LARVAL STRUCTURES (FIRST PERIOD) 



The neural tube, first of the embryonic systems to fold off, 

 is relatively advanced in development and exposed dorsally. 

 The brain vesicles are still simple and undistended. A marked 

 flexure is seen in front of the midbrain bringing the forebrain 

 vesicles down to a more ventral level against the roof of the 

 pharynx. The eye and ear vesicles are still rather compact 

 and unexpanded. Figure 1 for this first stage of R. palustris 

 shows well how the shape and surface contours of the head 

 are determined dorsally by the cerebral vesicles. The hind 

 brain and cervical cord are clearly seen, though the rest of 

 the cord lies deeper, hidden in side view by the dorsal margins 

 of- the lateral muscles. 



These muscle segments on either side of the notochord form 

 the main paraxial mass of the short tail and extend forward 

 in the body as far as the ear vesicles. They thus form the 

 middle zone of structures and are now broadly exposed later- 

 ally, since the dorsal region of the body cavity has not yet 

 grown up to overlap them. 



The ventral regions (fig. la) are simple, unmodified, and, 

 at the surface, uncovered by overgrowths of connective tissue 

 or cavities. This makes conspicuous the rounded surface of 

 the dilating pharynx. The submaxillary region is a flattened 

 area under the thyroid gland with two large suckers at the 

 sides. A short oesophagus follows, and back of this the liver 

 has begun to separate as a quadrilateral body at the anterior 

 end of the yolk mass around the middle line. It lies just be- 

 hind the systemic heart. Back of the liver the gut is tubular 



