234 PROFESSOR MARSHALL AND EDWARD J. BLES. 



The anterior limbs are present as small solid rounded processes 

 arising from the body wall opposite the head kidney, and projecting 

 into the opercular cavity close to its dorsal angle. They are at this 

 stage about the same size as the rudiments of the posterior limbs, or 

 very slightly smaller. 



The lungs are much larger than before, their actual size, however, 

 varying very greatly in different specimens ; in some cases they 

 extend almost to the hinder end of the body cavity, along the dorsal 

 surface of which they lie. The two lungs open in front into a median 

 laryngeal chamber, which communicates with the pharynx by a slit- 

 like glottis, strengthened laterally by cartilaginous bars. 



The thyroid body has lost its connection with the floor of the 

 mouth, and now forms a group of pigmented cells lying close to the 

 anterior wall of the pericardial cavity, and nearly divided into right 

 and left halves by a downwardly projecting process of the basihyal 

 cartilage. 



The oesophagus is now open, though its lumen is narrow, and in 

 some specimens very irregular. The precise time and the mode in 

 which the lumen is re-established present some curious and inte- 

 resting modifications, which we are unable to deal with fully here. 

 In nearly all 10 mm. specimens we find the oesophagus solid along 

 part of its length; at 11 mm. its condition varies very greatly in 

 different specimens ; and at 12 mm. the lumen is always present. 

 The lungs re-acquire their opening into the pharynx quite indepen 

 dently of the oesophagus, and at a slightly later period. The head 

 kidneys are still very large, and the Wolffian bodies are just beginning 

 to form. 



3. The Heart and Pericardial Cavity. 



The pericardial cavity communicates with the coelom by a pair of 

 apertures dorsal to the Cuvierian veins ; i.e., the posterior wall of 

 the pericardium is complete dorsally in its median part, but is per- 

 forated laterally by the apertures in question, which are the same as 

 the dorsal apertures of communication described in earlier stages. 



In the heart, the principal changes are the more marked separation 

 of the several cavities from one another ; the formation of the 

 auricular septum, dividing its cavity into right and left auricles • 

 the structural modifications in the wall of the ventricle, whereby it 



