DEVELOPMENT. 661 



having been completed towards the centre, although it still proceeds at 

 the periphery, the superficial layer of the blastoderm becomes a layer of 

 columnar nucleated cells, and the lower layer consists of larger masses 

 indistinctly nucleated, still granular and rounded, irregularly disposed. 

 In the segmentation cavity are the supplementary segmentation masses 

 or formative cells. 



When the egg is incubated, rapid changes take place in the blasto- 

 derm, resulting in the formation first of all of two, then of the three 

 layers, which have been already mentioned in the first chapter. The su- 

 perficial layer, or Epiblast, does not at first enter into these changes, 

 but continues to be a layer of nucleated columnar cells. But in the 

 lower layer of larger rounded cells, certain of the cells become flattened 

 horizontally, their granules disappear, and the nuclei become distinct. 

 A membrane of flattened nucleated cells is then formed, first of all 

 towards the centre of the area, afterwards peripherally also: this is the 

 Hypoblast. Between the two layers some cells, not belonging to either 

 layer, remain. These cells are almost entirely at the back part of the 

 area. The formations of the intermediate layer of mesoblast is more 

 complicated, and will now be described. 



At this period it is necessary to return to the surface view of the 

 blastoderm. Before incubation it is seen to consist of a more or less cir- 

 cular transparent area, the area pellucida, surrounded by an opaque 

 rim, which is called the area opaca. The area opaca rests upon 

 the white yelk: beneath the area pellucida is a cavity containing fluid. 

 In the centre of the area pellucida is a white shining spot, or nucleus of 

 Pander, shining through. The nucleus of Pander is in the upper dilated 

 extremity of the flask-shaped accumulation of white yelk upon which 

 the blastoderm rests. 



The yellow yelk consists of spheres 25 /* to 100 // in diameter, filled 

 with highly refractive granules of an albuminous nature, and the white 

 yelk being distinguished from the yellow not only by its lighter color, 

 but also because its vesicles are smaller than those of the yellow. Each 

 contains a highly refractive body. Some large spheres contain a number 

 of spherules. Some of these are vacuolated. The white yelk not only 

 envelopes the yellow yelk in a thin layer, and merges with the central 

 flask-shaped mass, already mentioned, but also is found in the yellow 

 yelk, forming with it alternate layers. 



Except that the central shining opacity of the pellucid area has dis- 

 appeared, that the size of the area has increased, and that the opaque 

 area has also increased, no other change can be remarked up to the for- 

 mation of the two complete layers. There is, however, a slight ill-de- 

 fined opacity at the posterior part of the area pellucida, known as the 

 embryonic shield. This opacity is probably due to the intermediate 

 cells already mentioned as existing between the epiblast and hypoblast. 



