l64 PRINCIPLES OF EMBRYOLOGY 



changing smoothly into the other as one goes from the dorsal midline 

 towards the sides. 



The process we have described so far would give rise to only two layers, 

 an outer which is the ectoderm and an inner which is the mesoderm. 

 Within this again lies the endoderm, and we have as yet said little as to 

 how it gets there. As a matter of fact, it is the endoderm which starts the 

 whole movement, since the early blastopore lies wholely within it. At 

 this stage, the invagination movement of the endoderm consists in the 

 withdrawal inwards of the main bodies of some of the large endoderm cells, 

 a process which can only be seen in sections (Fig. 20.13, P- 444). A little 

 later the withdrawing endoderm is followed by the first mesoderm flowing 

 round the dorsal lip. As this movement of the mesoderm spreads to more 

 lateral regions, making the lateral lips of the blastopore, the edge of the 

 mesoderm separates from the endoderm to form a free margin such as is 

 pictured in Fig. 9- 10; only in the mid-dorsal line is the comiection per- 

 manently retained at the anterior of the archenteron, and here it has 

 never been easy to decide where to draw the boundary between meso- 

 derm and endoderm. The separation only works round slowly to the 

 ventral side, with the gradual spreading of the blastopore lips. Meanwhile, 

 the region of endoderm which has become free of the mesoderm behaves 

 as though it were sucked in to the interior, folding inwards and at the 

 same time elongating along the dorsal meridian to keep pace with 

 the mesoderm to which its anterior end is still attached. Along 

 the dorsal surface of the endoderm a groove appears, at first shallow, but 

 gradually becoming deeper. This is the primitive gut, or archenteron. 

 As is clear from a section (Fig. 9.9) its walls and floor are made of endo- 

 derm, but its roof is at first mesoderm — in fact, the mid-dorsal mesoderm 

 which will later diflerentiate into the notochord. 



Gastrulation is often said to be completed by the time the blastopore is 

 reduced to a small sht. Although this is not actually the case, as can be 

 seen from Fig. 9.10 (see also p. 263), it will be as well to pause in our accoimt 

 of it to notice some of the more obvious changes which begin to occur at 

 this time. When the yolk-plug finally disappears from view, the egg is 

 still spherical in shape, evenly coloured all over with the darker tint 

 characteristic of the ectoderm, and diversified only by the blastopore sht, 

 which is elongated in the meridian of the dorsal axis. Fairly soon after this, 

 the first signs can be seen of the differentiation of the ectoderm into the 

 neural system and the skin. The neural area lies immediately in front of the 

 blastopore, and first appears (for instance in the newt, in which it is well 

 exhibited at this stage) as a pear-shaped or dumbbell-shaped area of some- 

 what darker colour. The edges of tliis area soon become elevated as ridges, 



