2-i CHARACTERS OF BLASTODERMIC LAYERS. 



have applied the terms primitive ectoderm and primitive entoderm. Since the middle layer 

 is developed from one or both of these primitive layers their permanent representatives are 

 morphologically different, having- lost the elements which go to form the middle layer, and it 

 'is therefore convenient to accentuate this distinction by the adoption of different terms to 

 represent the permanent layers. 



The generalisation that the formation of a bilaminar blastoderm is typically produced by 

 the invagination of a hollow spherical unilaminar blastodermic vesicle is due to Haeckel, and 

 was based largely upon the important researches of Kowalevsky, especially those on Sagitta 

 and Amphioxus. The process of delamination which in some animals produces the two 

 primary layers was originally regarded by Ray Lankester as the typical mode of formation, 

 but is now generally admitted to be a secondary modification. Finally, it has been shown 

 (Balfour, Lankester, R. and 0. Hertwig), as is set forth below, that the ccelom or body cavity 

 is typically developed, not by a process of splitting of the mesoblast (although in some 

 animals this may occur as a secondary modification), but as hollow protrusions from the 

 primitive alimentary cavity, the cells which bound these protrusions forming the mesoblast. 

 Thus from an originally single blastodermic layer by successive processes of invagination 

 or folding, the three permanent lamina? are ultimately produced. 



Such folds may be regarded as formed mechanically by local hypertrophic multiplication 

 of the cells of the laminee, an increased surface being thus found for the increased number of 

 cells. In analogous manner the folds which accompany the formation and separation of the 

 body and the development of the several organs, c.y., the nervous system, alimentary canal, 

 amnion, may also be regarded as resulting mechanically from cell-multiplication. This 

 mechanical theory of development was first enunciated by Pander, and has of late years been 

 applied extensively by several embryologists, notably by His (Entwickl. d. Huknchens, 1868, 

 and Unsere Korperform, 1874), and Rauber. 



Characters of the blastodermic layers. The three layers of the blastoderm 

 show from the first distinctive characters (fig. 26). The outer layer, or epiblast, is 

 epithelial in nature and consists of somewhat irregularly columnar cells closely set side 



liy 



Fig. 26. TRANSVERSE SECTION THROUGH THE FRONT END OP THE PRIMITIVE STREAK AND 



BLASTODERM OP THE CHICK. (From Balfour.) 

 pr, primitive groove ; m, mesoblast ; ep, epiblast ; Jiy, hypoblast. 



by side, forming a single stratum for the most part, except near the middle line, and 

 "becoming thinner and flatter towards the margins of the embryonic area. 



The inner layer or hypoblast is also epithelial, but the cells are at first all 

 flattened, and appear therefore quite thin and linear in sections of the blastoderm. 

 At a later stage, the hypoblast cells become markedly columnar and enlarged, so 

 that they considerably exceed the epiblast cells in size. 



The middle layer, or mesoblast, which differs, as we have seen, in its mode of 

 origin, being formed secondarily from one or both of the primary layers, also differs 

 from them entirely in its appearance and structure. Instead of consisting of cells 

 closely joined together into a continuous membrane after the manner of an 

 epithelium, the mesoblast is at first composed of cells which are not thus closely 

 arranged, but have, on the contrary, a considerable amount of intercellular fluid 

 between them. They are most irregular in shape, and are often branched and 

 united with one another, so that much of the mesoblast early resembles an 

 embryonic connective tissue. 



