SCHULTE AND TILNEY, NEURAXIS IX THE DOMESTIC CAT 321 



of the lumina, as well as models of the neural tube, were made in each 

 case and were found useful in controlling and interpreting the surface 

 relief. The period of development of the neuraxis covered by these 

 embryos extends from the appearance of the medullary plate to the 

 definition of the pallial anlage. It will be convenient, first, to record the 

 conditions observed in the several embryos and then proceed to a con- 

 sideration of the morphological questions outlined above. 



Description of Embryos 



Embryos Prior io the Appearance of Intersomitic Clefts. — (Plate 

 XXII, Fig. 1.) Toward the end of this period when the axial mesoderm 

 has thickened and is on the point of becoming segmented, the medullary 

 plate (-5)- is represented by an oval thickening of the ectoderm which 

 peripherally passes into the somatic ectoderm (7) by a gradual transition 

 {28) without clearly defined limits. In some of our embryos there were 

 irregular depressions and elevations of the surface, especially in its lateral 

 regions, which presented some resemblance to the Seitenfurche, Eand- 

 furche and Parietalzone of authors, but which, in view of the uniform 

 outlines of the excellent embryo here figured, we have not been able to 

 convince ourselves were natural structures. 



Embryos of One Somite. — (Plate XXII, Fig. 2.) In these embryos 

 one pair of complete intersomitic clefts is present. In addition, the 

 axial mesoderm presents on each side two or three transverse constrictions 

 which are evidently due to the inception of somites. In the cat it appears 

 that several somitic constrictions are initiated simultaneously. In our 

 reckoning we have regarded only the complete clefts and the number of 

 somites assigned to the embryos of our series are estimated in terms of 

 complete clefts. In the two embryos which we have classed as having 

 one pair of somites there are, in addition, two or three pairs of con- 

 strictions, one pair of which is situated in front of the first complete 

 intersomitic cleft. The medullary plate (5) merges laterally into the 

 somatic ectoderm (7) by a gradual transition {28) as in the earlier 

 stages. Cephalad its axial region is depressed. We would call attention 

 to the fact that the summits of these incipient medullary folds corre- 

 spond to the middle zone of the medullary plate on each side. 



Embryos of Two Somites. — (Plate XXIII, Fig. 1.) The medullary 

 plate has the same general characters as in the preceding embryos. It 

 is very broad and passes by gradual transition {28) into the somatic 

 ectoderm (7) at the sides. Craniad the medullary groove is somewhat 



*The numerals in ItaUcs refer to the numbers of the leaders in the figures. 



