46 J. B. Johnston. 
When the neural plate rolls up into a tube (Fig. 2) pressure is 
exerted upon the archenteron by the brain portion of the neural 
tube and the effect is seen in a compression of the archenteric space. 
The two angles seen in sagittal sections are now more marked and 
are separated by a thickening or fold of the intervening entoderm. 
In the meantime the notochord and mesoderm are forming in the 
same manner as in selachians. In the stages represented in Figs. 2 
and 3 the notochord ends anteriorly in a median mass which is not 
separated from the entoderm and is continuous laterally with the 
mesoderm which is in process of splitting off from the entoderm. 
This median mass remains in continuity with the preoral entoderm. 
Ive, 2 lhe, 8% 
Fic. 2. Amblystoma punctatum, neuropore stage. The foregut is already 
compressed and the palestomal (p) and neostomal recesses are sharply 
marked. Borax carmine. 
Fic. 3. A. punctatum, after closure of neuropore. Borax carmine. 
I have indicated in an earlier paper (1903) that the mesoderm 
formation is somewhat delayed in the head of Amblystoma so that 
the mesoderm of the hyoid and mandibular arches is split off from 
the entoderm as separate rods of mesoderm. The median mass 
mentioned is continuous laterally with the mesoderm which forms the 
premandibular somite (Fig. 4, A and B). The splitting off of the 
notochord stops at the posterior border of this median mass and in 
later embryos the point at which the end of the notochord remained 
