BY JAS. P. HILL AND C. J. MARTIN. 57 



middle region of the mid-brain up to a short distance in front of 

 the tirst somite, the notochord lies below the keel of the medullary 

 plate and is connected with the entoderm by a thin cellular 

 filament (fig. 5). Somewhat anterior to the first somite the 

 notochord l^ecomes entirely free from the entoderm, and continues 

 in this condition to its posterior end where it passes into the head 

 process of the primitive streak. The notochord increases in size 

 somewhat after Ijecoming entirely free from the entoderm. It is 

 then distinctly rod-shaped, while anteriorly it is somewhat oval in 

 section. 



The notochord is relatively of very small size in Platypus. 

 Heape* has also noticed a similar condition in the Mole, and he 

 regards it as due to the very early appearance of the nervous 

 system. 



Mesoderm. 



The mesoderm is at this stage established as two lateral wings 

 distinct from the ectoderm and entoderm except at certain regions 

 in the axial line, viz. :— -At the anterior flattened-out end of the 

 notochord, in front of the blastopore in the region of the head 

 process, and behind the blastopore in the region of the primitive 

 streak. As already mentioned in the description of the surface 

 view of the embryo, the mesoderm lying on either side of the 

 anterior end of the embryo forms two sharply limited plates to 

 which we have given the name of mesodermal head plates as dis- 

 tinguished from the mesoderm of the rest of the body. 



The head plates of mesoderm (PI. ix., h.p. mes.) are lateral 

 thickenings on either side of the future fore- and mid-brains, and 

 show no signs of segmentation. Their outer contours are directly 

 continuous with the forward continuation of the outer boundaries 

 of the proto vertebral zones of mesoderm. Their very distinct 

 posterior boundary is not due to the entire disa.ppearance of 

 mesoderm at this point, l^ut to a very marked thinning of the 



Quart. Journ, Micros. Science, Vol. xxvii. 1887, p. 139. 



