l62 PRINCIPLES OF EMBRYOLOGY 



the process is actually quite different from anything which had been en- 

 visaged. The most fundamental type of gastrulation movement is a flow- 

 ing or streaming one, in which a piece of tissue is carried bodily into a new 

 situation. With the recognition of this fact, the older terms were seen to 

 be not very appropriate, and they have largely disappeared from the 

 literature. An exception may be made for the word 'invagination', which 

 originally meant a massive infolding of a sheet of tissue (such as was 

 described in the infolding of the endoderm in echinoderms (p. 82)), but 

 which is now often used to cover any process by which an originally 

 outer layer is moved into an interior position. 



The most obvious change on the surface of the egg itself during the 

 process of gastrulation is the growth, rounding up and fmal closure of the 

 blastopore. When it first appears, this is a small somewhat pigmented 

 depression lying beneath the equator within the endoderm. Fairly soon it 

 enlarges laterally to form a short groove. This continues to grow longer, 

 and its two ends curve round to a crescent shape, which passes on to a 

 horseshoe and then to a closed oval. Although the blastopore originally 

 lay wholly within the whitish endoderm, by the stage at which it has 

 closed up to an oval, the more pigmented tissue of the animal hemisphere 

 is found to be lying at its edge, so that the outside of the egg is completely 

 dark except for the light spot of yolky cells within the oval blastopore; 

 this is known as the *yolk-plug' and is a very characteristic and easily 

 recognisable feature. The history of the blastopore is, however, by no 

 means complete; it gradually contracts in area, drawing together above 

 the yolk-plug which is fmally covered over and hidden from sight. By 

 the time gastrulation proper is ended, and the first signs of the embryonic 

 axis are appearing, the blastopore has been reduced to a narrow slit which 

 runs in the direction of the embryonic axis. 



By following through the history of vital marks, we can see that much 

 more has been happening than the appearance of the blastopore would 

 lead one to expect. Marks made anywhere within the ring of prospective 

 mesoderm are seen to move down towards the edge of the blastopore, to 

 flow over it, and, as can be shown by later dissection, to move away from 

 it again underneath the surface. The lips of the blastopore are, in fact, not 

 fixed positions occupied by a definite tissue, but are mere structural 

 appearances showing where the flow of tissue along the surface turns 

 downwards towards the interior of the egg. This explains how the lip, 

 which was originally made up of the pale endoderm, later becomes lined 

 with the darker material of the animal hemisphere; the pale material has 

 already moved away inside, and the dark material has streamed down to 

 replace it. 



