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TEXTBOOK OF ZOOLOGY 



Polar ingression resembles invagination except in one particular. 

 In invagination the cells, whether few or many, move inward as a 

 definite layer. In ingression the cells move inwards, but not as a defi- 

 nite layer. They do not retain their mass formation, but move as 

 individual cells or as very small groups of cells and rearrange them- 

 selves into the form of a layer with an interior cavity. 



The fourth means of gastrulation is delamination, which means the 

 formation of a second layer, divided off from the primary ectoderm 

 because the spindles of the cells are directed at right angles to the 

 surface of the embryo. The second layer is split off, so to speak, from 

 the first. The net result of gastrulation is a two-layered embryo of 

 primary ectoderm and primary endoderm. 



Fig. 420. — Formation of the endoderm in the gastropod, Patella, by unipolar 

 ingression. (Reprinted by permission from Outline of Comparative Embryology 

 by Richards, published by John Wiley and Sons, Inc., after Patten.) 



The formation of mesoderm changes the two-layered embryo into 

 a three-layered or triplohlastic one. Obviously there are only two 

 sources from which the mesoderm can arise, primary ectoderm or pri- 

 mary endoderm, and when it has been produced, definite endoderm, 

 ectoderm, and mesoderm result. If the mesoderm is derived from 

 ectoderm, it is known as eeto-mesodenn. In general, ecto-mesoderm 

 may be looked upon as of temporary importance, often being lost 

 when larval metamorphosis is accomplished. Important embryolog- 

 ical questions are connected with the formation of this kind of meso- 

 derm, and it has a bearing also upon the phylogeny of different ani- 

 mal groups. If the mesoderm is formed from the endoderm, it is 

 known as endo-mesoderm, and is the type with which we are most 

 familiar in the animals usually studied in courses in embryology. 



