THE SCIENCE OF EMBRYOLOGY I7 



previously separated. By interactions between parts which have newly 

 come together, the composition of the embryo gradually increases in 

 complexity. Thus a region which has been determined very early, for 

 instance, by an ooplasmic segregation, may be brought into contact with 

 an as yet undetermined part, and exert some influence which causes that 

 part to develop into some defmite type of tissue. This type of process 

 plays a particularly important role in vertebrates. For example, in amphi- 

 bia there is an ooplasmic segregation of the so-called 'grey crescent' soon 

 after fertilisation, which enables that region to develop into meso- 

 derm; when during gastrulation, this future mesoderm is brought into 

 contact with part of the ectoderm, it causes the latter to develop into 

 neural tissue. In such cases the part which exerts a stimulus and thus 

 causes the other reactant to develop into some tissue, say A, is said to 

 'evocate' A. 



(3) Field action 



In very many embryological processes, the development of any given 

 point in a region of the egg depends on its relations with other nearby 

 points or on its position within the region as a whole. For instance, if, at 

 the beginning of gastrulation in the amphibia, a small piece of the meso- 

 derm is cut out, rotated through 180° and replaced, a perfectly normal 

 mesoderm may still be formed; the development of the rotated piece 

 has been brought into line with its surroundings. Again, if a large part, or 

 even half of the mesoderm is removed, the development of each point is 

 modified in relation to its position within the total amount which is still 

 left, so that again a normal embryo is formed. Such happenings are spoken 

 of as 'field phenomena'. The reference of this name is to physical field 

 theories, such as those of magnetism, gravitation and so on. The imphca- 

 tion is not, of course, that these physical forces are operating, but merely 

 that the biological events have the same general character as the physical 

 ones; in both cases there must be some activity spread throughout the 

 whole region occupied by the field, and distributed in an orderly graded 

 manner, so that in some parts the activity is strong, in others weak, with 

 intermediate strengths between. 



In some ways, field properties are complementary to those involved in 

 ooplasmic segregation. In the latter we are confronted with a small num- 

 ber of differences, usually sharply distinct from one another and each 

 confmed to a particular region; in the former with graded differences in 

 some property which spreads throughout the whole of a wide area. From 

 another point of view, the notion of fields is closely connected with that 

 of evocation, since when we say that the development of one point in the 



