438 PRINCIPLES OF EMBRYOLOGY 



involves, firstly, the determination of the plane of bilateral symmetry of 

 the egg, and then the appearance of regional differences within the 

 organiser (head organisers, tail organisers, etc.) and of a dorso-lateral field 

 within the mesoderm. Clearly the pattern formation and the morpho- 

 genesis are inextricably involved with each other, and can only be separ- 

 ated conceptually by roughly classifying some of the events as rather more 

 chemical in nature (and therefore related to pattern formation) and others 

 as more definitely physical (and therefore connected with morphogenesis). 

 For convenience of discussion we shall start by considering some of 

 the morphogenetic processes which various authors have postulated, 

 and shall then consider the nature and development of the patterns 



(p. 455). 



During gastrulation we are confronted with massive streaming move- 

 ments by which the tissues are moved from one place to another. One 

 hypothesis about the causation of such movement would be to suppose 

 that certain regions are undergoing more rapid growth than others and 

 that the tissue streams are due to the expansion of the growing regions. 

 However, careful studies, particularly by Pasteels (1942^, have shown that 

 in gastrulating embryos mitosis goes on at a more or less uniform rate 

 in all regions. Similarly Gillette (1944) has shown that differential mitosis 

 cannot be held responsible for the roUing up of the neural plate into the 

 neural tube in Amphibia. Thus differential growth, important though it 

 may be at later stages, is certainly not a major factor in the morphogenesis 

 of the gastrula-neurula stages. 



Again it might be suggested that a differential expansion of the cells 

 in certain regions, caused by the imbibition of water, might be the 

 underlying cause of the morphogenetic movements. It has been found, 

 hov/ever, that there is very little change in specific gravity of the 

 cells during neurulation, and there can thus be little inhibition of water 

 (Brown, Hamburger and Schmidt, 1941), so this hypothesis also is in- 

 adequate. 



Another type of process, which' could be postulated to account for 

 changes in cell shape similar to those which might be produced by differ- 

 ential absorption of water, is the formation of fibrous structures in the 

 internal cytoplasm. One might expect that if the cytoplasm becomes 

 fibrous in character, the fibres would tend to lie parallel to one another 

 and give rise to an elongation of the cell, or parts of the cell. Such parallel 

 orientation of fibres could, in favourable circumstances, be detected by 

 polarised light, since it should cause some degree of double refraction m 

 directions dependent on the sub-microscopic orientation. This undoubtedly 

 occurs in the mitotic spindle; and membranes, such as the nuclear 



