. A text-book of agricultural zoology. Zoology, Economic. DEVELOPMENT OF MAMMALIA. 427 which it remains thick. This thick mass of granular cells we caU the embryonic or germinal area. During the following few days changes take place in the inner layer. About five days after impregnation the cells of the embryonic area divide into two layers, the lower composed of flattened cells, and forms the hypoblast. During the subsequent day a middle layer. Fio. 202.—OvDM OF Babbit. 1, Optical section of a rabbit's ovum at a stage closely following segmentation; Bl, blastopore; Hy, inner layer; Ep^ outer
. A text-book of agricultural zoology. Zoology, Economic. DEVELOPMENT OF MAMMALIA. 427 which it remains thick. This thick mass of granular cells we caU the embryonic or germinal area. During the following few days changes take place in the inner layer. About five days after impregnation the cells of the embryonic area divide into two layers, the lower composed of flattened cells, and forms the hypoblast. During the subsequent day a middle layer. Fio. 202.—OvDM OF Babbit. 1, Optical section of a rabbit's ovum at a stage closely following segmentation; Bl, blastopore; Hy, inner layer; Ep^ outer layer. 2, Rabbit's ovum between 70-90 hours after impregnation: , cavity of blastodermic vesicle (yolk-sac); Bp, outer layer ; Hy, inner layer; , albuminous envelope. (After E van Beneden.) arises and fuses with the epiblast to produce the true epiblastic layer. These two layers form a circular embryonic patch on the ovum, which on the seventh day becomes oval. There ap- pears at the lower part of this spot a primitive streak as in the fowl. The area then becomes pyriform, and the primitive streak is marked by a groove. The epiblastic cells which form the streak also give rise to part of the mesoblast. Very similarly to what we saw in the development of the chick the medullary groove arises in front of the primitive streak. The medullary plate between the two medullary folds is composed of one row of epiblastic cells, only, at first. The mesoblast arises from a double origin—(1) from the epiblast and (2) from the hypoblast. The notochord arises as a. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Theobald, Frederick Vincent, 1868-1930. Edinburgh and London, W. Blackwood and sons
Size: 2288px × 1092px
Photo credit: © The Book Worm / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1800, bookdecade1890, booksubjectzoology, bookyear1899
