PRE-CHORDAL PLATE AND CEPHALIC PROJECTION 449 



area, but it does possess the autonomous power to elongate into a slender 

 column of cells. 



I. Exogastrulation 



It was demonstrated by Holtfreter ('33) and also by others that embryos 

 may be made to exogastrulate, i.e., the entoderm, notochord, and mesoderm 

 evaginate to the outside instead of undergoing the normal processes involved 

 in emboly (fig. 216). For example, in the axolotl, Ambystoma mexicanum, 

 if embryos are placed in a 0.35 per cent Ringer's solution, exogastrulation 

 occurs instead of gastrulation, and the entodermal, mesodermal and noto- 

 chordal areas of the blastula lie outside and are attached to the hollow ecto- 

 dermal vesicle. The exogastrulated material, therefore, never underlies the 

 ectodermal cells but comes to lie outside the neural plate and skin ectodermal 

 areas of the gastrula (fig. 216B). 



Therefore, the phenomenon of exogastrulation indicates strongly that the 

 presumptive, neural plate and epidermal areas of the late blastula and early 

 gastrula are dependent upon the normal gastrulative process for their future 

 realization in the embryo. Exogastrulation also clearly separates the parts of 

 the forming gastrula which are concerned with emboly from those which are 

 moved by the forces of epiboly. That is, exogastrulation results when the forces 

 of epiboly are separated from the forces normally concerned with emboly. 

 Normal gastrulation is concerned with a precise and exact correlation of these 

 two sets of forces. 



J. Pre-chordal Plate and Cephalic Projection in Various Chordates 



It is evident from the descriptions presented in this chapter that the initial 

 invaginative movements in gastrulation begin in the region of the dorsal lip 

 of the blastopore in Amphioxus, fishes, and Amphibia. This initial movement 

 of cells in the region of the dorsal lip consists in the projection forward, toward 

 the future head region of the embryo, of foregut entoderm, pre-chordal plate 

 mesoderm, and notochordal cells. The foregut entoderm, pre-chordal meso- 

 derm, and the anterior extremity of the notochord come to lie beneath the 

 anterior portion of the neural plate. The complex of anterior foregut entoderm 

 and pre-chordal mesoderm lies in front of the anterior limits of the notochord 

 — hence, the name pre-chordal plate. As such it represents, as previously 

 observed, a part of the head organizer (see p. 401 ), the complete organization 

 of the vertebrate head being dependent upon anterior chordal (notochordal), 

 as well as pre-chordal, factors. 



In higher vertebrates a different situation prevails during gastrulation. As 

 observed in Chapter 7, the late blastula consists of a lower hypoblast and 

 an upper epiblast in a flattened condition, the hypoblast having separated 

 from the lower parts of the epiblast. The separation of the hypoblast occurs 

 shortly before the gastrulative rearrangement of the major, presumptive, organ- 



