ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 829 



an interval of about one or two hours between fertilization and the 

 appearance of the first two segmentation-spheres ; in the other forms 

 from 5-8 hours. After describing the process of segmentation in full, 

 and discussing the results of earlier observers, Dr. Ussow passes to 

 the next step, in which the blastoderm, &c., are developed. In the 

 germinal disk it is possible to distinguish (1) the central portion, 

 (2) the median portion, or area opaca, more or less ring-shaped in 

 form, and (3) the lower protoplasmic portion, not yet differentiated 

 into cells and continued as far as the lower pole of the egg. The 

 central portion is formed by a single layer and consists of small, 

 polygonal cells derived from the division of the six primary and two 

 secondary cleavage spheres. In the fresh condition the finely 

 granular protoplasm and the sharply contoured nuclei are quite 

 transparent. The cells are almost all of the same size (0"016 mm.), 

 the peripheral ones being alone somewhat larger. At first flattened, 

 they gradually become cylindrical ; and frequently alter in form by 

 dividing longitudinally. The cells of the area o'paca are longer, im- 

 equal in size, and polygonal in form ; there are only two or three 

 concentric rows ; they owe their origin to the multiplication of those 

 cleavage-cells which had been separated off by the development of 

 the equatorial groove. They are dark in appearance, owing to the 

 consistency of their protoplasm, and the thickness of the layer. The 

 broadest and lower portion consists at one time of 32 segments, which 

 are frequently arranged in pairs ; as there is not a single large 

 cleavage-cell, but 2-6 cells at the thickened apex of each segment, the 

 edge of the germinal disk is irregular and villous owing to the pro- 

 jecting angles of the cells ; between each pair of segments there is a 

 clear intermediate space, filled up by an extremely thin layer of the 

 formative yolk ; this disappears as the blastodermic cells multiply. 

 A little later (36th hour) there appear the rounded cells of the 

 mesoderm ; these arise from the cells of the median portion, which 

 undergo transverse division ; each of the cells so formed is rounded, 

 and gradually takes on a cylindrical form. As soon as these cells 

 appear the process of division begins to affect all the cells of these 

 parts of the germiual disk, and is effected either transversely or lon- 

 gitudinally. Three or four successive rows of the larger blastoderm- 

 cells, forming the median portion, divide longitudinally as soon as 

 they have divided transversely ; this, of course, increases the breadth 

 of the median portion, which also becomes a thicker and therefore a 

 darker ring ; this ring surrounds the unilaminate and still transparent 

 central portion. The other six days of the first developmental period 

 are occupied by the multiplication of the cells of the peripheral 

 portion of the germinal disk ; the upper aud median genninal layers 

 extend over the surface of the nutrient yolk. 



At the end of the second day of development the middle layers 

 consist of several rows of cells ; at the same time the ectodermal cells 

 have continued to undergo transverse division, and have thus narrowed 

 the central portion of the germinal disk. On the third day, separate 

 groups of mesodermal cells make their way into the central portion, 

 and towards the end of that day the upper limits of the mesoderm 



Ser. 2.— Vol. II. Z 



