EMBRYOLOGY. 
129 
and Germinal spot, lying upon the yellow yolk of the 
unlaid egg.* Whatever view be taken of the relations of 
the eggs of the Vertebrata, the important point to be 
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noticed is that the embryo of a fish, batrachian, reptile, bird, 
or mammal, including man at an early stage of life, is a 
guitar-shaped body (Figs. 177, 167), consisting of three 
membranes lying over one another, and narrowly bound 
together (Fig. 168); and if we were ignorant o: the animal 
whose egg had been transformed into such a body, it would 
be very often impossible to say what would result from its 
development. These membranes are called blastodermic, 
or tissue germinating from the organs of the future animal 
growing in them.T 
The skin and central nervous system are developed in 
the External, or upper membrane; the osseous, muscular, 
vascular, reproductive, and urinary systems, the walls of 
the alimentary canal, and its appendages, are produced in 
the Middle membrane; while the epithelium, which lines 
the alimentary canal and its appendages, the lungs, liver, 
etc., is derived from the Internal or lower membrane. 
In speaking of the Primitive trace, at page 127, we called 
attention to the furrow known as the Primitive groove. As 
development proceeds, this furrow deepens, and ir the 
* We have called attention to the distinction of white and yellow yolk, as 
the white yolk, or part of it, is supposed by Peremesko to form = Middle 
layer of the chick; his view being that the balls of the white yolk, by 
amcebiform movement, pass up and between the External and mc 
blastodermic membranes, coalesce, and so form the Middle membrane. 
Physiologists the embryo is stated as consisting of two layers, 
the External and Internal germinal layers, or blastodermic membrane, from 
which the future animal is developed. The view of the embryo consisting 
of three germinal layers was distinctly enunciated by Remak as long ago as 
1852, “ Comptes Rendus," tome xxxv., and even earlier. Since that time 
Remak's views have been confirmed by Rathke, Kölliker, Stricker, Waldeyer, 
Klein, and others. We, therefore, give in the text what may be called the 
German theory of Embryology. 
