DEVELOPMENT OF THE EMBRYO 



447 



which is to form the blastoderm proper, and the narrow denser area 

 known as the periblast, which forms the outer margin. The periblast 

 forms a continuous layer over the yolk, peripherally. 



In the center of the germ disc, the first cleavage furrow appears. 

 (Fig. 257). It is short and shallow, running about one-half the diameter 

 of the disc. We do not even know whether the first cleavage extends 

 directly through the central portion of the embryo. The main embry- 

 onic axis lies almost at right angles to the long axis of the whole egg, 

 the head end of the embryo being directed toward the left when the 

 sharp end of the egg is held pointing away from the observer. The first 

 cleavage plane does not seem to have any definite relation to either of 

 these axes. 



The second cleavage is also vertical, and almost at right angles to 

 the first, so that we have four adequal cells, all, however, incomplete. 

 The third cleavage appears about an hour after the first. This is usually 

 parallel to the first. It divides the disc into two rows of four cells each. 

 This cleavage may be quite irregular in form, and from now on it is im- 

 possible to tell exactly how and when, in relation to time especially, 

 these egg cells divide. Consequently, after they have divided and formed 

 sixteen cells, all of these cells are very irregular, and there is a tendency 

 in the fourth cleavage plane to separate the eight cells into a central and 

 a distal one. 



' Fig. 258. 



Diagrams showing the blastulae : A, of Amphioxus ; B, 

 of frog, and C, of chick ; D, blastodermic vesicle of mammal. 

 (After Semon.) 



A group of central cells now becomes circumscribed, but these must 

 not be confused with the marginal cells which remain incomplete both 

 below and distally, retaining their connection with the periblast. These 

 central cells have been separated by a horizontal cleavage plane, and 

 this cleavage plane separates the more superficial cellular elements from 



