INDIVIDUATION — FORMATION OF PATTERN AND SHAPE 443 



material does so. Holtfreter (1933) has described the process in detail. He 

 showed that the regions of the neural system developed in such partial 

 exogastrulae correspond very closely w^ith the extent of the mesoderm 

 that is formed inside as opposed to outside, a fact w^hich provides a neat 

 illustration of the developing pattern of anterior and posterior organisers, 

 which we shall shortly discuss (p. 455) (Fig. 20.12). 



Figure 20.13 



A, section through the future blastopore region of the axolotl, at a time 

 when the invagination is indicated only by condensations of pigmented coat 

 to which early flask cells are attached. B, semi-diagrammatic section through 

 a gastrula, to show the flask cells lining the blastopore and archenteron. C, 

 the endoderm cells forming the floor of the archenteron at a later stage ; 

 cells which have broken contact with the surface have contracted their 

 'necks' into small pigmented lumps. (From Holtfreter 1943.) 



There are at least two factors involved in the in-turning mechanism. 

 The most obvious is the formation of peculiarly shaped cells at the posi- 

 tion at which the blastopore first appears. These are usually described as 

 flask or bottle cells. The main cell body, in which the nucleus is situated, 

 is roughly ovoid and is drawn out into a long, thin neck by which it is 

 connected to the external surface in the blastopore region. The narrow- 

 ness of the neck may be partly the result of the contraction of the coat 

 over the site of the blastopore furrow, but it is probably also influenced 

 by processes going on within the cell cytoplasm, since the neck region, 

 from which yolk granules are absent, has been found to show double 



