THE SCIENCE OF EMBRYOLOGY 7 



Stage 5. Gastmlation. In a short and extremely critical period of develop- 

 ment, the various regions of the blastula become folded and moved 

 around in such a way as to build up an embryo which contains three more 

 or less distinct layers (only the imier and outer layers appear in coelenterates 

 and lower forms). These three fundamental layers are known as (i) the 

 ectoderm, which lies outermost, and will develop into the skin and the 

 neural tissue, (ii) the endoderm, which lies innermost and will form the gut 

 and its appurtenances, and (iii) the mesoderm, which lies between the other 

 two, and will form the muscles, skeleton, etc. The foldings by which 

 these layers are brought into the correct relation with one another are 

 very different in different groups, as they are bound to be since the blas- 

 tulae from which they start may not have the typical spherical shape, 

 particularly when there is much yolk in the egg. But in spite of differences 

 in the process of gastrulation, the situation to which it leads — one in which 

 there is an outer, an inner and a middle layer — is rather uniform in all 

 groups. 



Stage 6. Formation of the basic organs. Soon after gastrulation the funda- 

 mental pattern of the embryo begins to appear. In most cases, the organs 

 wliich arise are ones which will persist throughout the remainder of 

 development, and will form the most essential organs of the adult animal; 

 but in some animals the embryo at first develops into a larva, forming 



Neural lube 



Somite 

 Nolochord 



Lateral 

 mesoderm 



Figure i.i 

 To illustrate the basic structure of a generalised vertebrate embryo. 



