44 THE FIRST DAY. [CHAP. 



The changes which take place during the first day will be 

 most easily considered under three periods : from the 1st to 

 the 12th, from the 12th to the 20th, and from the 20th to the 

 2 fcth hour. 



2. From the 1st to about the 12th hour. — During this 

 period the blastoderm when viewed from above is found to 

 have increased greatly in size. The pellucid area, which at 

 the best is but obscurely marked in the unincubated egg, 

 becomes very distinct (the central opacity having dis- 

 appeared), and contrasts strongly with the opaque area, 

 which has even still more increased both in distinctness 

 and size. 



For the first few hours both the pellucid and opaque 

 areas remain circular, and the only change, besides increase 

 in size and greater distinctness which can be observed in 

 them, is a slight ill-defined opacity or loss of transparency, 

 which makes its appearance in about the middle of the 

 pellucid area. This is known as the embryonic shield. 



3. Slight as are the changes which can at this stage be 

 seen from surface views, sections taken from hardened 

 specimens bring to light many most important changes in 

 the nature and arrangement of the constituent cells. 



It will be remembered that the blastoderm in the un- 

 incubated egg is composed of two layers, an upper (Fig. 3,ep.) 

 and an under layer; that the upper is a coherent membrane 

 of columnar nucleated ceils, but that the lower one (Fig. 3,/.) 

 is formed of an irregular network of larger cells in which 

 the nuclei, if present, are rarely visible ; and that in addition 

 to this there are certain still larger cells, called ' formative 

 cells ' (Fig. 3, b), lying at the bottom of the segmentation- 

 cavity. 



Under the influence of incubation changes take place 

 very rapidly, which result in the formation of the three 

 layers of the blastoderm. 



The upper layer, which we shall henceforward call the 

 epiblast (Fig. 10, A), takes but little share in these changes. 



In the lower layer, however, certain of the cells begin 

 to get flattened horizontally, their granules become less 

 numerous, and a distinct nucleus makes its appearance in 

 them ; the cells so altered cohere together and form a mem- 

 brane (Fig. 10, C). The membrane thus formed, which is. 



