I Q2 GENERAL PRINCIPLES OF ZOOLOGY. 



this way: first, embryonic cell-material is marked off into 

 a special aggregation, usually by infolding (prganological 

 differentiation], and then later, this aggregation becomes 

 changed into tissue (histological differentiation]. The way 

 in which this happens differs in the various animal groups. 

 The following is the most general mode : from the ectoblast 

 arise the skin with its glands and appendages, the nervous 

 system, and the sensory epithelium ; the cntoblast gives 

 rise to the most important part of the digestive tract with 

 its most essential glands ; while muscles, blood, supporting 

 and connective substances, excretory organs, in whole or in 

 part, arise in the meso blast ; the sexual organs are also 

 usually mesoblastic. 



Relations of the Germ-layers in Budding. Often 

 of late the question has been raised as to how far the germ- 

 layer theory is applicable to the occurrences in asexual 

 reproduction. At first, one would expect in budding, and 

 still more in the case of division, that each organ of the 

 daughter animal would be split off from the corresponding 

 organ of the maternal animal, or, if that is made impos- 

 sible by conditions of space, from a mass of tissue belong- 

 ing to one of the same germ-layers. In many instances, 

 this is certainly the case, as, for example, in the budding of 

 Hydroids the entoderm and ectoderm of the bud arise 

 from the corresponding layers of the maternal body (Fig. 

 87). But, through recent investigations, exceptions to this 

 rule have become known. In Bryozoans and Tunicates 

 there are undifferentiated cells which are employed in 

 cases of budding; these are elements not yet furnished 

 with the characteristics of a definite body-layer which, in- 

 dependently of the position which they assume in the ma- 

 ternal animal, can be employed, according to need, in the 

 the building up of organs. 



5 . The Different Forms of Sexual Development. 



Embryonic and Postembryonic Development. - 



While the occurrences described are going on (fertilization 

 and cleavage of the egg, formation of the germ-layers) 



