440 Leland Griggs. 



one at a time. The embryos were first divided roughly into 

 the conventional stages such as yolk plug stage, open neural 

 plate stage. These stages, however, owing to the fact that the 

 markings of the open neural plate are very transitory, had to 

 be subdivided. 



Some difficulty was encountered in thus subdividing the prin- 

 cipal stages owing to a remarkable variation in the younger 

 embryos. Some of the grooves and infoldings were so deep 

 that abnormalities seemed at first sight to be very common. 

 Further study of these variations showed that they were not 

 promiscuous abnormalities but variations along certain well 

 defined lines. Those structures which in a majority of embryos 

 were faintly marked, and in a few embryos were not seen at all, 

 were very plainly marked in others. The deeply furrowed 

 specimens were, therefore, the most valuable for study when 

 it could be shown, as it usually could without difficulty, that the 

 deep furrows correspond in extent and position with the faint 

 furrows in other eggs. Patten has already pointed out in his 

 study of Limulus embryos the principle that in the process of 

 recapitulation those embryos which vary from the normal aver- 

 age type may show best of all some features of the ancestral 

 condition. This seems to be especially true in a study of neuro- 

 meres. Some embryos seem to show no neural segmentation 

 in the early stages, the majority show it faintly, a few show it 

 very distinctly, but in all cases the number of neuromeres is the 

 same where they show at all and the size of the neuromeres is 

 approximately the same. Hence we are obviously not deal- 

 ing with a meaningless variation, but we are justified in mak- 

 ing a careful study of the most clearly sculptured neural 

 plates. 



A second difficulty in the way of dividing the embryos into a 

 series of stages is found in the fact that several distinct struc- 

 tures are forming on the surface of the egg at the same time and 

 with a relative rapidity which varies with the different eggs. 

 For example, if the eggs are assorted according to the closing of 

 the blastopore the stages will not show a consecutive develop- 

 ment of the neural plate, for among the eggs with a wide open 



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