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26 EARLY EMBRYOLOGY OF THE CHICK 



This region where the blastoderm is thin and free from the yolk 

 marks the position of the blastopore. 



Gastrulation begins with the undertucking of the cells at the 

 free margin of the blastoderm. Figure 7, ^, is a diagrammatic 

 surface view of the blastoderm represented as a transparent 

 object. The position and the extent of the invaginated ento- 

 derm seen through the overlying ectoderm are indicated by 

 the cross hatched area. The appearance of the blastopore 

 locates the caudal region of the future embryo and permits the 

 definition of its longitudinal axis. This axis is indicated by the 

 line h-h on Figure "], B. A diagram of a section cut in the 

 longitudinal axis and passing through the blastopore of an 

 embryo of this stage is shown in Figure 7, E. The invaginated' 

 cells which constitute the entoderm form a layer extending 

 cephalad from the thickened lip of the blastopore. The yolk 

 forms the floor of the gastrocoele. Figure 7, C, is a diagrammatic 

 surface-view of a later stage in the same process. The extent 

 of the entoderm is marked by cross-hatching as in the diagram 

 of the previous stage. The undertucking of the cells at the 

 blastopore has ceased by this time, and as indicated in Figure 

 7, C. by the black area, and in Figure 7, i^, by the solid mass of 

 cells seen in section, the blastopore has become closed. 



During the entire time that the process of gastrulation has 

 been in progress there has been constant cell proHferation going 

 on in the blastoderm as a whole. The growth of the blastoderm 

 has been evidenced especially by increase in its surface extent 

 which has resulted in a general spreading of its peripheral mar- 

 gins over the yolk. This extension has taken place uniformly 

 at all parts of the margin except in the posterior quadrant where 

 the blastopore is located. Here the cells proliferated, instead 

 of spreading out over the yolk have turned in at the lip of the 

 blastopore to form the invaginated entoderm. This particular 

 part of the margin of the blastoderm, having contributed the 

 cells formed in its growth to the entoderm which grows back 

 toward the center of the blastoderm, takes no part in the 

 general peripheral expansion. As a result the blastopore region 

 is, as it were, left behind and the rapidly extending margin of 

 the blastoderm on either side sweeps around and encloses it. 

 The blastopore at the time of its closure thus comes to lie 

 within the recompleted circle of the germ wall (Fig. 7, C). 



