926 



OP GENERATION EMBRYONIC DEVELOPMENT. 



Fig. 327), and with this a distinction becomes apparent between the upper 

 and lower layers of the segmentation masses. Those of the upper or super- 

 ficial series assuming a columnar form, and presenting well-defined nuclei, 

 arrange themselves side by side, and constitute a kind of membrane (Fig! 



FIG. 325. 



A represents the earliest stage. The first furrow, 6, has hegun to make its appearance in the centre 

 of the germinal disk. In B the first furrow is completed right across the disk, and a second similar 

 furrow nearly at right angles to it has appeared. The disk thus becomes divided somewhat irregularly 

 into quadrants by four (half) furrows. In a later stage (C) the meridian furrows (6) have increased in 

 number from four, as in B, to nine, and cross furrows have also made their appearance. The disk is 

 thus cut up into small central (c) and larger peripheral (d) segments. Several new cross furrows are 

 seen just beginning, as ex. gr. close to the end of the line of reference (d). 



326, a) ; whilst those of the lower series remain larger (>), have no nucleus, 

 or only an indistinct one, and form rather a close irregular network than a 

 distinct membrane. A few of the cells of this layer, constituting the so-called 

 formative cells (bd, Fig. 327) becoming detached, fall to the bottom of the- 

 segmentation cavity, and rest on the white yolk. This is the usual condition. 



FIG. 326. 



Section of the Germinal Disk of a Fowl during the later Stages of Segmentation. The section, which 

 represents rather more than half the- breadth of the blastoderm (the middle line being at c), shows that 

 the upper and central parts of the disk-segment faster than those below, and at the periphery. In the 

 majority of segments a nucleus can be seen, and one is probably present in all. The segments contain 

 a number of highly refracting spherules. In the central part of the blastoderm the upper cells have 

 commenced to form a distinct layer. No segmentation cavity is present. , large peripheral cell ; 6, 

 larger cells of the lower parts of the blastoderm; c, middle line of blastoderm; e, edge of the blastoderm 

 adjoining the white yolk ; w, white yolk. 



of the blastoderm at the time that the egg is laid. The future development 

 of the chick takes place entirely in the area pellucida. In the course of a 

 few hours after being laid the blastoderm of the fecundated egg, in conse- 

 quence of the flattening of the lower cells of the mesoblast and the develop- 

 ment of nuclei in their interior, presents a division into three layers (Fig. 



