144 WALTER HEAPJil. 



in Fig. 39, only the notochordal mass is itself considerably larger the 

 lateral ingrowth of the hypoblast is here also indicated. 

 Fig. 41. From still further posteriorwards : the notochord is not yet 



isolated from the hypoblast, but formed into an arc. 

 Fig. 42. From the hind end of the embryo, immediately in front of the 

 primitive streak : the notochord is a large thickened axial mass, with 

 no indication of the growth of the hypoblast below it. 

 Fig. 43, Stage H. — A transverse section through the medullary cord of 

 an embryo with eleven protovertebrse, from the region in front of the first 

 protovertebra and behind the hind-brain. Between the lateral mesoblast 

 plate and the cord is a small space, in which several nuclei are seen. The 

 space is continuous with blood-vessels in process of formation, and the nuclei 

 show a tendency to pass into the medullary cord. One such nucleus is shown 

 in the drawing. 



Figs. 44 and 45, Stage J. — Transverse sections through the hind-brain of 

 an embryo with fourteen protovertebrse. The alimentary canal is narrow in 

 front (Fig. 44), and wider posteriorly (Fig. 45). The two grooves in the 

 epiblast on the under surface in the anterior section converge in a single deeper 

 groove in the posterior section, where the fusion of the epiblast and hypoblast 

 takes place, and where the mouth will eventually be formed. The dorso- 

 ventral elongation of the fore-gut and the notochord is due to the plane in 

 which the section was cut, caused by the cranial flexure. The presence of 

 mesoblast cells between the notochord and the floor of the brain is to be 

 noticed. 



Fig. 46, Stage J, is a section, not completely transverse, through an embryo 

 with fourteen protovertebrse, passing through the hind-brain and the auditory 

 involution. The flrst aortic arch is shown on one side, and a lateral prolonga- 

 tion of the fore-gut to form the first visceral cleft on the other side. 



Fig. 47, Stage J. — Transverse section through an embryo with thirteen 

 protovertebrse in the region of the second protovertebra. Fore-gut crescent 

 shaped. Anterior cardinal veins and dorsal aortse present. Embryo is com- 

 pletely folded off from the yolk-sac. The heart is enclosed in the peri- 

 cardium. The thick outer wall and flattened epithelial layer of the heart are 

 here seen to be connected by fine processes of the cells forming one or other 

 of these layers. 



Fig. 48, Stage J. — A transverse section through the primitive streak of an 

 embryo with fourteen protovertebrse. The thickened lateral mesoblast will be 

 seen, by comparing this section with that drawn in Fig. 50, to be concerned in 

 the formation of the allantois. Allantoic vessels (all. v.) are to be seen in this 

 section. 



Fig. 49, Stage J . — A median longitudinal section through the head of an 

 embryo with fourteen protovertebrse. The division between the fore- and 

 mid-brains and the folded floor and thin roof of the hind-brain is shown. The 



