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.

    102/374 (page 68)
    scribed above for Birds. Posteriorly the raesoblast arises where the epiblast and hypoblast are fused at the primitive streak, and clearly owes its existence to both. In the region in front of the primitive streak the mesoblast is proliferated from the hypoblast as two lateral masses which posteriorly unite with the above- mentioned mesoblast. There also appears to be an actual con- tinuity between the developing notochord and the dorsal portion of the paired mesoblast. There is some diversity of opinion amongst other investigators concerning the origin of the mesoblast amongst Mammals. It may be concluded that the Mole, being an Insectivore, would probably not have a very specialised development for Fio. 67.—Sections THRonaH the Blastoderm of a Mole (Talpa). [After Heape.] A. Longitudinal section through the middle line of part of an embryonic area in which the primitiTS streak has commenced to form ; the blastoderm is per- forated in front of the primitive streak. B. Transverse section through the middle of a well-developed primitive streak; the epiblast and mesoblast are fused, but the hypoblast is distinct; the mesoblast here extends beyond the embryonic area. C. Same as B. but through the hind-knob of the primitive streak. All the layers are fused in the embryonic area, but are distinct beyond. bp. blastopore; ep. epiblast; hy. hypoblast; m. mesoblast; p.sk. primitive streak. a Mammal, and, for the present, the above statement may be regarded as holding good for Mammalia generally. When the embryology of the Prototheria (Ornithodelphia) is investigated, it will doubtless be found to resemble that of the Lizard in many points, and will demon- strate that any peculiarities in the development of Mammals is due first to the presence, and secondly to the subsequent loss, of food-yolk. Although in most Vertebrates the mesothelial mesoblast is at first solid, it very shortly splits into two layers, a peripheral or somatopleur, and a visceral or splanch- nopleur (figs. 59, 71). The pleuro-peritoneal cavity or coelom thus produced is strictly homologous with the persistent body-cavity of such forms as have hollow archenteric diverticula. It is evident from the foregoing summary that the derivation of the true body- cavity or ccelom from archenteric diverticula occurs in one or more examples of