254 PROFESSOR MARSHALL AND EDWARD J. BLES. 



ductive organs. The allantoic bladder forms a saccular diverticulum 

 from the ventral surface of the rectum as in the adult. 



The lungs lie close against the dorsal wall of the body, and extend 

 almost its whole length. 



The brain has all the parts of the adult, and very nearly the adult 

 proportions. In the skeleton, ossification is advancing rapidly, and 

 the limb girdles and sternum are already well developed. 



The two halves of the thyroid body, which have now separated 

 completely from each other, lie in close contact with the anterior 

 wall of the pericardial cavity. Each half is a mass of vesicles of 

 irregular size and shape, lined by a single layer of cubical epithelial 

 cells, and with the cavities of the vesicles filled by a coagulable fluid. 

 The thyroid body receives blood from the thyroid branch of the 

 lingual artery, and returns it by a short vein to the inferior jugular 

 vein. 



The subcutaneous lymphatic spaces are enormously developed, 

 especially in the sides and floor of the head. 



3. The Heart and Pericardial Cavity. 



The pericardial cavity still communicates with the ccelom by a 

 pair of apertures near its dorsal surface, situated below and at the 

 sides of the median laryngeal chamber. 



The sinus venosus is much as before. Of the vessels in com- 

 munication with it the posterior cardinal vein has diminished greatly 

 in size, and a subclavian vein is present returning blood from the 

 fore limb. 



The aperture from the sinus venosus to the right auricle is now 

 guarded by a valvular fold. The left auricle has increased in size, 

 but is still a good deal smaller than the right. Both auricles have 

 their walls thickened by muscular strands, which form interlacing 

 reticular ridges on their inner surfaces. Near the outer sides of the 

 auricles some strands cross their cavities as in the ventricle. 



The ventricle is relatively larger than before. The internal net- 

 work of interlacing muscular strands is closer and more intricate 

 than before, but the true outer wall of the ventricle still remains 

 thin, as thin indeed as the wall of the auricles. The muscular 

 reticulum is absent from the centre of the ventricle, where a wide 

 open channel is left, leading from the auricular aperture to that 



