CHAPTER I. 



PORIFERA. 



Sponges reproduce by sexual and non-sexual means. To 

 the non-sexual kinds of reproduction belong — (1) sprouHnr} 

 or budding, which may lead to the formation of complicated 

 stocks or colonies ; (2) the formation of small buds which 

 separate from the parent body and grow up independently 

 into new individuals ; (3) reproduction by means of 

 gemmulce. 



The investigations on the development of sponges from 

 the fertilized egg have not up to the present time yielded 

 a uniform plan for the embryology of this group, and they 

 frequently contradict one another. The following may be 

 mentioned as features common to the development of all 

 sponges. ^ 



(1) The sexual products 

 arise in the connective 

 tissue of the so-called 

 mesoderm out of cells 

 which at first are not to 

 be distinguished from the 

 connective tissue cells of 

 this layer. 



(2) The eggs are not 

 surrounded by any cuti- 

 cular envelope (chorion) 

 or vitelline membrane'. 

 They lie naked in a cavity 

 lined, with endothelium 

 (Fig. 1 e) in the mesoderm of the parent body. Here the 

 expulsion of the polar globules, fertilization, and early de- 

 velopment take place. 



13 



Fig. 1.— Egg of Placina trilopha in the 

 parent body (after Magdeburg), r, polar 

 globules ; e, endothelial lining. 



