128 



J. THOS. PATTERSON. 



D. Study of Sagittal Sections. 

 The results of the above series of experiments are in them- 

 selves sufficient to prove the position taken in this paper. But 

 inasmuch as some writers have drawn their conclusions from a 

 study of sections, it seems advisable to introduce a few figures 

 to show that sections support the above contention. By far the 



jri ^-rrrrn 1 1 : I m in i i n III li i/ll/ / 1 JM i /77l // //////77777m-, — -D 



Fig. 6. Shows the first mesodermic cleft, and there are also indications of the 

 second and third clefts, r, beginning of the rudimentary somite. X 2 47- 



Fig. 7. Introduced to show the relation of the first and second clefts to the rest of 

 the embryo. /, anterior end of the primitive streak ; b-p, region of differentiation of 

 the embryo. X 



it ~ h 



Fig. 8. Enlarged portion of the opposite side of the embryo represented in Fig. 

 7. s, shallow depression. X 2 47- 



Figs. 6-8. In these, as in the remaining figures of this paper, A is anterior and B 

 posterior end, and a, b, c, etc., are respectively the first, second, third, etc., clefts. 



most critical study made on sections is by Miss Piatt, whose view 

 is well summed up in her conclusion, in which she says : " My 

 conclusions are, therefore, that the first break in the mesoderm 

 occurs anterior to the first protovertebra, and that two protover- 

 tebrse (or,, more correctly, one and a half) are slowly formed 



