426 MORGAN. [Vol. X. 



the corresponding section of older embryos. This would 

 give data to determine whether the elongation of the em- 

 bryo was due solely to axial concentration of the earlier embry- 

 onic region, or due to other processes. For, obviously, if the 

 areas of the older sections were less than the areas of the cor- 

 responding younger sections, an axial elongation must have 

 occurred to accommodate the mass. Practically, the difficulties 

 in the way of such comparisons are very great, and the chances 

 of error very large. I have, therefore, not attempted to make 

 such comparisons in detail. The only safe method by which 

 to get at the exact data will come, I think, from accurate 

 reconstructions of such embryos. Then, the relation by 

 weight between the various parts and sections can be readily 

 made. 



A study of all of these sections shows nevertheless most 

 clearly one important result. The areas of the sections through 

 the posterior end of the embryonic shield of the earlier stages, 

 are much greater than the areas of those sections of older em- 

 bryos, taken at the same absolute distance from the head. It 

 follows that all of the material of the posterior part of the 

 younger stages is not needed to form the same portion (i.e., 

 the portion at the same distance from the anterior end,) of the 

 older embryo. Two processes must then take place simulta- 

 neously in the embryo as it elongates posteriorly: ist, an axial 

 concentration ; 2d, a transfer backwards of a large part of the 

 material. 



In the older stages a third process takes place, to assist in 

 the elongation, viz., an axial elongation of the whole embryo. 

 The embryo elongates as the cells press in toward the central 

 longitudinal axis. (See stages shown in Figs, ii and 12.) 



We may next turn our attention to a study of the germ-ring. 

 From a theoretical point of view there are three phases in the 

 growth of the germ-ring : i st, a period of formation out of the 

 blastoderm; 2d, the growth of the germ-ring to the equator of 

 the ^%'g — a period of continuous increase in its length; 3d, 

 the period of growth from the equator to the closure, and dis- 

 appearance of the ring. During the last period it is continually 

 getting shorter. We see from the surface that the breadth of 



