Experimental Studies on Germinal Localization. 5 



the characteristic sensory hairs of the apical organ; and even into 

 what I believ^e to be muscle-cells and mesenchyme-cells, though, 

 unlike the foregoing cases, the precise origin of these was not 

 traced. These eggs thus represent the opposite extreme to such 

 forms as those of Amphioxiis, the echinoderm, or the nemertine, 

 and give a result which, apart from the hypothesis of qualitative 

 nuclear division, agrees essentially with Roux's original conception 

 of mosaic-development, with the conclusions of many students 

 of cell-lineage, with the experimental results of Crampton on 

 the gasteropod-egg, and with those of Fischel regarding the 

 ctenophore-egg.^ 



11. 



PRELIMINARY OBSERVATIONS ON THE UNSEGMENTED EGG AND 

 THE NORMAL DEVELOPMENT. 



The egg of Dentalium, like that of Cerebratuliis, possesses 

 certain features by means of which the axis may be determined 

 in the living egg from the moment of its release from the ovary. 

 The egg is more or less deeply pigmented, perfectly opaque, and 

 of a color that varies in different individuals from light olivaceous 

 to reddish brown or almost brick red. When first set free the 

 egg is somewhat irregular, but quickly becomes more rounded. 

 It is then seen to be very considerably flattened, so as often to 

 be almost biscuit-shaped, one side being always more flattened 

 than the other, and often more or less Irregular In contour. 

 Viewed by reflected light the central region of each of the flat- 

 tened sides is seen to be occupied by a very distinct, though vaguely 

 bounded, white area, nearly or quite free from pigment (Fig. i) ; 

 these areas, as shown by the subsequent development, correspond 

 with the two poles of the egg, and the more flattened side, which 



1 The eggs of Patella, which were employed mainly for a study of the iso- 

 lated blastomeres, were available from the middle of March until the latter 

 part of May. Those of Dentalium, which were used especially for the develop- 

 ment of egg-fragments, first became mature at the beginning of June, when 

 less than two months remained for their study. The shortness of this period 

 accounts for some of the obvious gaps in my work. The complexity of the 

 subject, and the practical difficulties presented by the material are such that 

 more extended work, with additional material, will be required for its com- 

 pletion. 



