250 PROFESSOR MARSHALL AND EDWARD J. BLES. 



The thymus forms a very conspicuous body in transverse sections. 

 It consists (Fig. 15, T) of a pair of rounded masses of closely com- 

 pacted spherical cells, lying in the sides of the roof of the pharynx 

 below the anterior part of the auditory capsules. 



One of the most marked features of the present as compared with 

 the earlier stages, is the enormous development of the subcutaneous 

 lymph spaces. These form sacs of literally enormous size (Fig. 

 15, Y), filled with a coagulable fluid, and almost completely surround- 

 ing the sides and ventral surface of the body and head. It is owing 

 to these lymph spaces that spirit specimens of tadpoles in these later 

 stages almost invariably have a shrivelled appearance. 



3. The Heart and Pericardial Cavity. 



The pericardial cavity has the same communications as before 

 with the ccelom. 



In the heart itself there are no great changes. The left auricle is 

 larger relatively than at 12 mm., but is still much the smaller of 

 the two. Owing to the position of the inter-auricular septum, the 

 left auricle lies somewhat obliquely behind the right one. 



The auriculo-ventricular aperture is much narrowed by valvular 

 folds. The ventricle (Fig. 15, H V) is in much the same condition 

 as before : its outer wall is if anything rather thinner than that of 

 the auricles, except at the places where the muscular trabecule arise 

 from it. These are now thicker and much more numerous than 

 before. They are arranged so as to leave a central cavity, free from 

 trabecular, and leading from the auriculo-ventricular aperture to the 

 aperture into the truncus arteriosus. The rest of the cavity of the 

 ventricle has the appearance of a spongy reticulum, owing to the 

 number of the trabecular and the intricate way in which they cross. 

 This mode of formation of the musculature of the ventricle by an 

 internal reticular mesh-work, in place of solid thickening of its walls, 

 allows the blood, which occupies all the meshes of the sponge-work, 

 to come into immediate relation with every part of the ventricle, and 

 so explains the absence of nutrient vessels in the ventricular walls, 

 first noticed by Hyrtl. 



The truncus arteriosus has thick muscular walls, and contains 

 valves arranged as before, there being as yet no arrangement for 

 regulating the distribution of the blood from the two auricles to the 



