SOCIETY OF THE UNIVERSITY OF ABERDEEN. 101 



extends from a point behind the bucco-pharyngeal membrane, to end in 

 the caudal region just short of the hindgut and between the cloaca and 

 medullary plate, and not extending into the tail. The chorda lies in 

 close relation with entoderm forming the roof of the intestine, and only 

 toward its caudal termination is it quite separated off, and here lies in 

 the mesoderm midway between the gut and the medullary plate. At 

 its cranial commencement the chorda is at first not separated off, but 

 appears more as a heaping up of entoderm cells in the roof of the 

 pharynx ; soon, however, it becomes more differentiated, as represented 

 in Fig. 2. At the level of the first pair of mesodermic somites the 

 chorda appears more as an evagination of the entoderm of the root of 

 the gut, and this condition obtains almost to its caudal end, where it 

 gets quite separated off from entoderm (Fig. 3). The cells of the chorda 



,-;% WO*' 



&; ^i^" 



Fig. 2. Notochord in region of first pair of mesodermic somites, 

 and Fig. 3 in caudal region. 



En., entoderm of gut; Ch., notochord ; Sf.c., spinal cord ; Ao., dorsal aorta. 



are large, oval and clear, and contain small, distinctly staining nuclei ; 

 in some of the cells mitosis is seen. There is no trace of a cuticular 

 membrane. 



VASCULAR SYSTEM. 



Cardinal Veiny. The anterior cardinal veins, containing blood- 

 cells, are present in the region of the first to the fifth mesodermic 

 somites. A connection with the heart cannot be established from a 

 careful examination of the sections. No trace of posterior cardinal 

 veins is to be seen. 



Vitdlwe Veim. Although in the wall of the yolk sac are numer- 

 ous blood-vessels containing blood-cells, still the vitelline veins are 

 difficult to trace. Their terminal parts can be defined where they lie 

 in the septum transversum on the ventral aspect of the gut, and open 



