SCHVLTE AXD TILNEY, NEURAXhS IX THE DOMESTIC CAT 327 



the medullary folds. The anterior iieuropore is reduced to a minute ori- 

 fice situated ventrally at about the middle of the optic vesicles, thus 

 affording, in comparison with the foregoing embryo conclusive evidence 

 of progressive closure at the anterior lip of the neuropore and to this de- 

 gree depriving the orifice of value in determining the mor])hologic ceph- 

 alic extremity of the neural tube. Apart from this small opening the 

 anterior neuropore is closed, but the ectoderm is adherent over the cranial 

 portion of the optic vesicles and the adjacent region of the thalamen- 

 cephalon. The midbrain shows no sign of division into two segments. 



Embryos of Tirelve and Thirteen Somites. — (Plate XXXIII.) These 

 embryos form a closely graded series, passing from the conditions de- 

 scribed in the embryos of ten somites to those attained by the fourteen 

 somite embryos of our series. In the region of the anterior neuropore 

 (<5) they show a considerable degree of variation in the closure. In em- 

 br3'^o Xo. 534 of twelve somites, the neuropore has been completely closed. 

 The ectoderm is, however, adherent at the middle of the sagittal length 

 of the optic vesicles and further over the region of junction of optic vesi- 

 cle (1) and thalamencephalon (16). In embryo No. 86 of thirteen 

 somites, the ectoderm is adherent in the whole length of the optic vesicle 

 and there are three small orifices, one at the middle of the optic vesicle, 

 oile at its junction with the thalamencephalon and one in the thalamen- 

 cephalon itself. 



There are three oblique segments in the hindbrain; their interseg- 

 mental constrictions give attachment to the profundus (Sa), quintal (3) 

 and acoustico-facial {J^) ganglia in the order named cranio-caudad. The 

 acoustico-facial is continuous with the ganglionic crest (20) which ex- 

 tends for somewhat more than half the length of the neuraxis. Follow- 

 ing these oblique ganglionic segments is a series of vertical segments; 

 their constrictions corresponding to the mesodermic somites are six to 

 seven in number. It is thus seen that the vertical segments of the neu- 

 raxis correspond in location to the somites, but lag considerably behind 

 them in number, which we take to mean that an interval in time elapses 

 between the formation of the mesodermic somite and formation of the 

 corresponding myelomere. It seems to us, therefore, that ontogenetically 

 myelomeres are secondary to the mesodermic somites. The three oblique 

 hindbrain segments, associated with the three large ganglia, are situated 

 in advance of the somites. We would emphasize the difference in their 

 disposition as evincing their independence of the myomeric segmentation. 



Embryos of Fourteen Somites. — (Plate XXXIV.) The two embryos 

 of this stage in our series show a close correspondence in the neuraxis, 

 save only that embryo Xo. 548 is in most respects slightly in advance of 



