An introduction to the study of embryology / by Alfred C. Haddon.

  • Haddon, Alfred C. (Alfred Cort), 1855-1940.
Date:
1887
Catalogue details

Licence: Public Domain Mark

Credit: An introduction to the study of embryology / by Alfred C. Haddon. Source: Wellcome Collection.

Provider: This material has been provided by University of Bristol Library. The original may be consulted at University of Bristol Library.

    80/374 (page 46)
    epiblast. ^ The covering cells, however, soon disappear, either entering into the formation of the embryonic epiblast or become attached to the decidua (see p. 92); in the latter case they would not form any portion of the embryo proper. A translucent circular patch next appears at what corresponds with the upper pole of other oosperms (fig. 42), this embryonic area soon becomes ovoid and is homologous with the area pellucida of the Fowl. A primitive streak with its groove makes its appear- ance at the posterior end of the area. In the Mole, according to Heape, the blastoderm is perforated immediately in front of where the primitive streak is commencing to form (fig. 43, a); later this spot is marked by a small down-growth of the epiblast, which really corresponds with the anterior border of the blastopore. Somewhat more posteriorly a complete fusion takes place between the epiblast and incipient mesoblast (fig. 43, b), while at the pos- Fig. 42.—Rabbit's Oosperm Seven Days after Impregnation. 3.47 mm. in length. Side view deprived of its envelopes. Magnified about 10 diameters. [From Kblliker.] 0.(7. Area pellucida, or embryonic area; ge. in- ferior limit of the hypoblast; below this line the blastoderm consists solely of a siugie layer of epiblast. terior end of the streak a complete fusion of all the layers occurs (fig. 43, c); but the three layers are distinct beyond the streakltself. The similarity of a Mammalian blastoderm at this stage with that of a Bird, or especially of a Lizard, is very striking, and it led Balfour to propose the view that the Mammalian ovum originally possessed a large quantity of yolk, since the blastodermic vesicle is clearly homologous with the yolk-sac and contains a coagulable fluid comparable to some extent with the yolk. The primitive streak is the same structure in both Sauropsids and Mammals, that is, it represents a vanished blastopore. It has since been proved by Haacke and Caldwell that the pre- viously known but discredited fact was true that the Monotremata are oviparous, and that the eggs are in all essential points per- fectly comparable with those of Keptiles. Thus Balfour's deduction from purely embryological data has been verified.