42 YOUNG TWIN HUMAN EMBRYOS WITH 17-19 PAIRED SOMITES. 



young embryos it ought to give evidence of a seam, but no evidence whatever of any seam 

 or fusion is to be found in this part of the brain. It does not seem reasonable to consider 

 this region a part of the dorsal wall of the brain, when in all its relations it is so different 

 from all the rest of the dorsal portion of the nervous system, and at this stage of develop- 

 ment it also seems to me quite rational to take the presence of the seam at the line of fusion 

 of the lips of the neural groove and the connection with the ectoderm as a criterion for 

 determining what is the dorsal surface. Otherwise one is forced to the conclusion that the 

 extreme anterior end of the brain closes very early and independently of the ordinary clos- 

 ure, the anterior neuropore being the space temporarily existing until these two regions 

 of closure fuse. If this is the case this anterior end must close very much earlier, indeed, 

 than the rest, for it is already separated by mesoderm from the ectoderm. The conclusion 

 does not seem a reasonable one to me, and I also do not see any reason why, at this early 

 stage of development, any part of the roof of the brain should be rotated down until it 

 lies in line with the floor. The brain at this stage is a simple tube with dilatations repre- 

 senting the anlagen of its various parts, but there are no structural peculiarities that would 

 indicate a displacement of the anterior end of the roof to the ventral surface. I think 

 there is every ground to assert with Johnston (1910) : "Rather, as the neural plate rolls up, 

 the neural tube tapers to a point in the preoptic recess, and the lamina terminalis is the 

 anterior part of the seam of closure along the mid-dorsal line." 



CONCLUSION. 



In conclusion I wish to briefly call attention again to the following points: These 

 embryos are twins and show appearances and degrees of development which are almost 

 identical throughout. Yet these embryos are not identical or "real" twins, as shown by 

 the fact that they were not contained in one sac, but each embryo had its own individual 

 membranes. These twins, therefore, are the product of the fertilization of two ova simul- 

 taneously or with only a very small interval between. This gives an opportunity for more 

 variation than is possible with twins which are the product of one ovum, and produced by 

 independent development of each cell of the 2-celled stage into an embryo. Yet in spite 

 of this they show a remarkable correspondence. Two embryos more alike would be hard 

 to find. 



It is to be emphasized that these embryos are extremely valuable in bridging the large 

 gap existing between embryos described to date, with 14-15 pairs of somites, and those of 

 22-23 pairs. That there may be no hesitation in accepting the work embodied in this 

 paper, I can positively assert that there is no evidence or hint of any pathological condition 

 in these embryos. They are in excellent condition for topographical work, but minute 

 histological work on the cells can not be pursued on account of the slight damage resulting 

 from the section cutting and from the unavoidable beginning of maceration before the 

 embryos were extruded from the uterus. This damage, however, in no way interfered with 

 the study of the form and relations of the various organs and parts of the body. 



As my final word I wish to tender to Professor J. Playfair McMurrich my most sincere 

 thanks for the loan of these valuable embryos and for furnishing me with material and all 

 available facilities for prosecuting this study. It would be unfitting to close without also 

 referring to his live, keen interest in the progress of this work and his pleasure in the results 

 it has been my fortune to attain. 



June 1, 1914. 



