COMPARATIVE EMBRYOLOGY 811 



There are several processes leading to the formation of endo-meso- 

 derm, but it always arises from a group of cells lying at the sides of 

 the gut and gradually extending themselves to make a mass which 

 later hollows out to form the mesoblastic somites. 



Enterocoele formation is the evagination of a series of pockets from 

 the cavity of the primitive gut and occurs, for example, in amphioxus. 

 The third type of coelome formation is by a solid outgrowth of cells 

 arising from the enteron, a common method among the vertebrates. 

 Lastly, is mesenchymous coelome formation, the formation of meso- 

 blastic somites by the fusion of scattered mesenchymous cells. This 

 is rather rare and of little importance in general embryology. 



Mesoderm formation results in a larva consisting of three germ 

 layers and completes the second great period of embryological devel- 

 opment. From the three germ layers the different organs of the body 

 are developed by invagination, evagination, foldings, and the reten- 

 tion of embryonic thinness in certain regions while neighboring areas, 

 because of differential growth, thicken and become elaborated into 

 the organs as we know them; this latter process is especially well 

 illustrated in the development of the brain. 



In many animals development is direct, the embryo passing directly 

 from a stage of organ formation into a small, but adultlike creature. 

 Others, especially those which develop in the sea and the terrestrial 

 insects, undergo a process of metamorphosis, their earlier adaptations 

 to one type of habitat being insufficient to fit them to live in the dif- 

 ferent habitats of the adult. A small organism essentially like the 

 adult is the result, and only the processes of histological differentia- 

 tion and of growth are necessary to produce the mature organism. 

 Organ formation and histological differentiation complete the proc- 

 esses which we commonly speak of under the heading of embryology. 



