284 MR. G. BEOOK ON THE 



rim of the blastoderm, the axis of the embryo terminating 

 posteriorly on its anterior edge. This pore is ultimately seen to 

 close up, or at least apparently, and nothing left but a thickened 

 patch. This blastodermic pad is, as may be seen from the 

 account of its formation, composed of all that remains of the 

 thickened rim of the blastoderm after it has encircled the yolk, 

 and will, as pointed out by Kupffer, His, and E.yder, enter 

 largely into the formation of the tail, which we shall presently 

 see sprout out in continuation with the axis of the embryo. As 

 Byder points out, this blastodermic pad may be considered a 

 true tail-swelling. At the time of closure of the blastopore 

 nine or ten protovertebrge are formed, and the tail end of the 

 embryo is somewhat spathulate in shape. The widened part in- 

 cludes the notochord, with an unsegmented plate of mesoblast 

 on each side. It is still impossible to trace the notoehord to the 

 extreme posterior edge of the embryo. 



We have now brought our consideration of the development of 

 the embryo up to a stage when there are nine or ten proto- 

 vertebrae. The invagination of the eye-lenses has progressed but 

 slowly. The auditory vesicles, however, have been fully closed in, 

 and the nasal pits are formed. The brain is now divided into the 

 three embryonic regions, and a lumen has appeared in all three, 

 although one does not show itself in the spinal cord until 

 later. 



The optic bulbs have become fully differentiated and the 

 lumen has appeared. 



Kupffer's vesicle has nearly reached its maximum development 

 and the pigment-spots are well established. 



Sect. 3. From Closure of the Blastopore to fhe Pulsation 

 of the Seart. 



The first important developments after the closure of the 

 blastopore are the formation of the heart and of the alimentary 

 canal, and both seem to arise at the same time, on the closure of 

 the pore or immediately after it. The heart begins as a small patch 

 often distinguishable by its light amber colour. The patch is 

 situated a little to the right of the central line of the embryo, as 

 seen ventrally, and almost beneath the hind brain, and was most 

 readily noticed on a side view. It has been found as early as the 

 stage when eight protovertebrse are formed (the blastopore being 

 closed), but is usually first distinguishable at a stage when there 



