GROWTH AND TRANSFORMATION 155 



heart is formed, and the spaces which arise in the spongy 

 mesoderm (mesenchyme) become extended and coalesce 

 to produce the enlarged blood-containing cavity (haemocoel, 

 see pp. 6, 35) characteristic of insects and of arthropods 

 generally. 



There is one important feature of the embryonic growth 

 as to which many insects seem to show a remarkable di- 

 vergence from animals generally. We have seen that the 

 germ-band early becomes differentiated into outer and inner 

 cell-layers. Of these, the outer (ectoderm) gives rise to 

 the skin and the nervous system as is the case in the vast 

 majority of animals. As a general rule the inner layer of 

 cells (endoderm) becomes the lining of the primitive 

 digestive cavity, and the mesoderm whence the muscular 

 and connective tissues are developed, appears as a derivative 

 of the endoderm close to its junction with the ectoderm. 

 Now in insects, and indeed in most Arthropods, the digestive 

 portion (mid-gut) of the food-canal is much restricted, 

 occupying only a relatively small section of the alimentary 

 tract, the extensive fore-gut and hind-gut being derived from 

 inpushed ectoderm and Hned with cuticle. It is certain 

 that in all insect embryos by far the greater part of the inner 

 cell-layer must be regarded as mesoderm, since from it the 

 muscles, and connective and blood tissues are formed, as is 

 typical in animals generally. The feature of insectan 

 embryolog}^ still most imperfectly understood is the origin 

 of the mid-gut, and it is not established if any definite 

 rudiment in an insect embryo can be identified as certainly 

 comparable to the endoderm among animals generally. 

 This is a surprising gap in our knowledge of development, 

 since the endoderm is one of the two " primary " germ- 

 layers, usually recognisable as a definite entity in a very 

 early stage of animal growth in the egg. 



Three principal alternative interpretations of the origin 

 of the mid-gut in insects have been given by the numerous 

 investigators of the subject. Many early students of 

 insect embryology regarded the yolk-cells as representing 

 the endoderm and giving rise to the mid-gut ; but in more 



