72 



TEXT-BOOK OF EMBRYOLOGY. 



was present from the time of its differentiation from the inner cell-mass (Fig. 52), 

 apparently disappears or is replaced by the protentoderm, as the latter grows 

 forward under the ectoderm and finally the protentoderm becomes continuous 

 at its anterior border with the yolk entoderm that remains. The area where the 

 two become continuous is the "completion plate" (Fig. 55). 



The disappearance of the yolk entoderm, or its replacement by protentoderm, 

 occurs, however, only in a linear area; that is, the protentoderm grows forward 

 only as a narrow band of cells which replaces a correspondingly narrow band of 



Mesoderm Blastopore 



Embryonic disk 



Chordal plate Completion plate 



Fir,. 55. — Medial section of germ disk of bat. van Beneden. 



yolk entoderm. And since this strip of protentoderm is destined to give rise to 

 the notochord, it is sometimes known as the "chordal plate" (Fig. 54, S 3 ). 

 From the manner of formation of the "chordal plate," it is continuous along 

 each side with the yolk entoderm (Fig. 54, S 2 ). 



No human ovum showing gastrulation has been observed. What is known 

 of the formation of the germ layers in man is discussed on p. 89. 



FORMATION OF THE MIDDLE GERM LAYER— MESODERM. 



Mesoderm Formation in Amphioxus. — In such a simple type as Amphi- 

 oxus the formation of the middle germ layer is readily observed and there is 

 consequently no question as to the manner in which it arises. In higher forms, 

 however, the origin of the mesoderm has been and 5till continues to be one of 

 the most difficult of embryological problems. 



In the two-layered Amphioxus gastrula the mesoderm first appears as two 

 symmetrical evaginations of the entoderm which push out dorso-laterally from 

 the archenteron (Fig. 56, a). That part of the entoderm which lies between the 

 two mesodermic evaginations is composed of somewhat higher cells than those 

 of the developing mesoderm and constitutes the anlage of the notochord (chorda). 

 The lips of the mesodermic evaginations next come together (Fig. 56, b) in such a 

 manner that the mesoderm becomes completely separated from the archenteron 

 (Fig. 56, c) . While this separation is taking place, the mesodermic evaginations 

 divide transversely into a number of segments which lie on each side of the 

 medial line and are known as the mesodermic somites — primitive segments 

 (Fig. 57). Meanwhile, the chorda anlage is being transformed into the chorda 



