68(3 



THE OVUM AXD BLASTODEEM. 



Kolliker. M. Barry, Bischoff, Coste, and Kemak. The knowledge of the develop- 

 ment of the ovum and embryo of mammalia was especially advanced in the suc- 

 ceeding decennial period by the valuable memoirs of Bischoff on the rabbit, dog, 

 guinea-pig, and roe-deer, published successively between 1847 and 1854, and an 

 important addition was made to the history of human development and that of 

 some animals by the publication of the elegant and elaborate work of Coste in 

 1847 and several following years. 



To the careful observations of Remak more particularly, as described in his 

 work on the development of the fowl and the frog, published in 1851-54, we owe 

 the fullest and most consistent account of the structiu-e and formation of the 

 blastoderm and of the relation of its several parts to the earlier phenomena of 

 embryonic development. The Lectures of Kolliker, published in 1861, formed 

 the most valuable addition to the history of development in the ten years suc- 

 ceeding the publication of the researches of Remak. In 1868 the blastoderm and 

 its early transformations were subjected to renewed examination in the elaborate 

 researches of His (Untersuch. lib. die erste Anlage des Wirbelthierleibes). In 

 the succeeding years appeared the varied researches of Dursy on the development 

 of the head, Waldeyer on the ovaries, Oellacher on birds and fishes, and Goette 

 on batrachia and birds, and numerous others, so that every year brings forth 

 numerous original contributions to different departments of the subject. In 

 1874 there appeared the first English treatise on the development of the 

 embryo since the time of Harvey, in the excellent " Elements of Embryology," 

 by Jil. Foster and F. M. Balfour — the latter of whom is also the author of im- 

 portant original researches quoted in the coui'se of this section. In the same 

 year a short and useful systematic work on the Embryology of Vertebrate 

 animals has appeared by Dr. Schenk of Vienna. 



Origin of the Mesoblast. — Although there is the general agreement before 

 indicated among embryologists as to the trilaminar structure of the blastoderm 

 in the ovum of the higher vertebrates, when it has made some progress in 

 development, and as to the general relation of the several layers to the produc- 

 tion of the systems and organs of the embryo, there is by no means the same 

 unanimity of views as to the manner in which the different layers, and more 

 especially the lower and middle layers, come into existence. 



Fig. 497. 





Fig. 497. — Microscopic view of a vertical section through the Blastoderm of 

 THE Bird's Egg after twelve hours op incubation. (From Strieker.) ^f" 



S, upper layer of cells or epiblast ; D, lower layer now forming a single continuous 

 layer of flat cells, or hypoblast ; M, large formative cells beginning to form the middle 

 layer, or mesoblast ; A, subgerminal part of the yolk. 



In the egg of the fowl the cells of the middle layer begin to make their appear- 

 ance in the central part of the blastoderm between the two original or primitive 

 layers from the eighth to the twelfth hour of incubation, while about the 

 same time a lower layer becomes distinct, as before stated, by the arrangement 

 in a single layer of the lowest cells, their assumption of the flattened form, 

 and their mutual union somewhat after the manner of a i^avement-like epithelium. 

 But while this is apparent towards the centre of the blastoderm there is 

 accumulated towards the periphery in a thickened zone (ojiaqiie area) a quantity 

 of cells of larger size and granular aspect in which no division into an under 



