GECOLOGY . -153 



21. By the invagination of the blastula the gaslnda or two-laj'ered 

 embryo arises. 



2 2. The gastrula contains the primitive digestive tract or archcnteron, 

 opening to the exterior through the blastopore; it consists of two epithelial 

 layers, the entoderm or the inner germ-layer, lining the archenteron, and 

 the ectoderm or outer germ-layer. 



23. Between the inner and the outer germ-layer still a third, the 

 middle germ-layer, mesoderm, may be formed. 



24. The middle germ-layer arises either by an infolding or a cutting 

 off of a part of the entodermal epithelium: epithelial mesoderm, mesothe- 

 lium; or by the migration of separate cells to form a gelatinous tissue 

 (mesenchyme) . 



25. Many animals deposit their eggs before or shortly after fertilization 

 (oviparous) ; others lay eggs which have already been fertilized in the 

 maternal body, and at the time of laying have passed through some of the 

 stages of development {ovo-vivi parous). A third series of animals give 

 birth to living young {viviparous) . 



26. The development of an animal is either direct or indirect (meta- 

 morphosis) . 



27. Indirect development or metamorphosis is where the young animal, 

 as it comes from the egg, differs from the sexually mature animal in two 

 points: {a) by the lack of certain organs which occur in the sexually 

 mature animals; (b) by the appearance of organs, larval organs, which are 

 lacking in the sexually mature animals. 



III. RELATION OF ANIMALS TO ONE ANOTHER. 



Just as between the organs of an animal there exists a regular relation 

 which is termed correlation of parts, so also the different individual 

 animals stand in intimate reciprocal relations to one another. The con- 

 ditions of existence of many animal species are altered, if other forms 

 appear or disappear, or undergo an extraordinary reduction or increase 

 in number of individuals. Such reciprocal effects are usually of a special 

 nature and can be understood only by individual study; a few are of wide 

 occurrence and are hence suitable for a general consideration; to such 

 belong colony and society formation, parasitism, and symbiosis. 



I. Relations between Individuals of the Same Species. 



Colony Formation. — Colony and society formation are relations 

 which exist between individuals of the same species. An animal colony 

 is a union of individual animals by an organic bodily comiection; the 



