114 CHARLES BROOKOVER 



hours until hatching, when the interval was gradually lengthened 

 until eleven days after hatching. Staining in bulk over night with 

 Delafield's hematoxylin diluted with nine times its bulk of water 

 was used in preference to iron hematoxylin because it does not 

 stain the yolk granules. 



We shall commence our description at the age of 112 hours 

 after fertilization, just after the head and tail have begun to rise 

 from the yolk and the embryo measures 8 mm. in length. A 

 section transverse to the neutral tube, slanting slightly backward 

 and upward reveals two thickenings in the ectoderm (fig. 1) 

 which develop into the nasal capsules and the olfactory nerve. 

 Hence the olfactory nerve may be said to be placodal in origin. 

 The depth of the placode is but little more than twice the thick- 

 ness of the adjacent ectoderm. The placode is rounded into 

 contact with the neural tube along a part of its deepest border. 

 This contact exists in four sections 6 micra in thickness. In 

 other sections mesodermal elements intervene between the placode 

 and the neural tube. Except for one small space in the next 

 anterior section of this embryo, the membrane separating the 

 placode from the neural tube is intact and everywhere as evident 

 as in the drawing. Consequently it is doubtful whether the 

 placode has any fibrous connection with the neural tube at this 

 age. Mitoses are shown in the center of the placode and other 

 karyokinetic figures are found in adjacent sections within the 

 placode near its contact with the neural tube, indicating the 

 region of cell proliferation at this stage of development. 



Other stages earlier than 112 hours reveal the same essential 

 structure except that the placode is not so thick. The whole 

 embryo is more compact and the mesoderm is not so reticular as 

 that dorsal of the placode (fig. 1) but more like that shown ven- 

 trally of the placode at 112 hours. The neural tube has the 

 usual structure of early embryos with its central layer of germinal 

 cells and its more external zone of nuclei of epithelial cells. Nei- 

 ther of these layers is thick and the external marginal velum is 

 scarcely recognizable. 



The next stage (fig. 2) is a little over 120 hours old. In the 

 eight hours intervening since the last stage there has been a large 



