302 PROCEEDINGS OF THE AMERICAN ACADEMY. 



ontogenetic advance. In these a large well developed bridge is 

 found, overlying a depression in the germinal disk below, to which 

 it is attached at only three points, one of these being usually larger 

 than the others, and apparently representing the posterior overgrowth, 

 which seems to develop slightly in advance of the lateral prolifera- 

 tions. An anomalous condition of the bridge is interesting. In 

 one embryo measuring 4 mm., with a germinal disk 0.28 mm. in 

 diameter, the lateral proliferations seem to have been entirely sup- 

 pressed, and we have an overgrowth from only one region, the 

 posterior, extending forward along the median line of the disk. 



The fate of the bridge seems to be, that its free anterior margin 

 finally meets the true ectoderm of the disk ; the structure then sinks 

 down until it comes in contact with the underlying ectoderm, with 

 which it finally fuses. At the same time the disk increases in area, 

 this being largely due to a rearrangement of the cells of the disk in 

 consequence of the addition received from the bridge. This method 

 of increasing in size at this stage was first suggested to me by the 

 fact that few nuclei are found in a karyokinetic condition. Accord- 

 ingly, in the case of the two germinal disks represented by Figs. 5 and 

 6, Plate I., which correspond to the two stages of development in 

 question, I counted the number of nuclei in each, as seen in sec- 

 tions, to determine the numerical relations of the cells. In the disk 

 of Fig. 5 I found 1067 cells, in that of Fig. 6, 992 ; although em- 

 bryos in their early development grow at very different rates, still 

 the facts which these numbers present, together with the absence of 

 nuclear figures, would seem to point to a simple rearrangement of 

 existing cells as the principal factor in the increase in area of the 

 germinal disk at this stage, rather than to an active multiplication 

 of cells. 



The oldest embryos considered in this paper consist, then, of a 

 blastodermic vesicle, composed of a continuous inner sac of entoderm 

 closely surrounded by a layer of ectodermal cells, which in the ger- 

 minal disk are thickened into a flat, ovate expanse, without jirimitive 

 groove or streak, with no signs of any mesoderm, and with a few 

 widely scattered " Deckschicht " nuclei on the extra-germinal area. 

 The entodermal cells are thickened in the region of the germinal 

 disk until they become nearly isodiametric, and they are also 

 thickened, though to a less extent, in an area all around the ger- 

 minal disk, the diameter of which is about three times as great as 

 that of the disk itself. With this summary I now pass to a consider- 

 ation of the observations of other investigators on the mammalian 



