us ClKXERAL rRIXCIPLES OF ZOOLOOV 



blastopore (A), thus parlialh' separaliiig a ])aii- of lateral chamliers from 

 the rest. IMie proeess continues; the lilaslopore closes, while the 

 eirtoJernial folds extend to the oj^posile side, where they fuse with 

 the walls (B). In this way a jjair of dioniir poinhcs are cut olT 

 from the rest of the archeiUeron whieli forms the lumen of the 

 digestive tract and its derivati\"es, while the walls of the pouches 

 form the mesotltelium, that of the digesti\-e region the secondary 

 entoderm. In each cadomic pouch t\\'o walls are recognizalile, an 

 inner or sphiiiiliiiic layer which unites with the entoderm to form the 

 wall of the di;;estive tract, the splainiuioplfiirt-, while the soniulir layer 

 unites similarly with ectoderm to form an outer hod)' wall, the soma- 

 toplfiirc. From the foregoing it is evident that the mesothelium is strictly 

 not a single layer, but consists of two layers which pass into each other, and 

 that its origin is closelv connected with the formation of the body cavity. 



Occurrence of Mesench5rme and Mesothelium. — There are purely 

 mcsencli)-matous animals, like the llat-worms, and purely mesothelial, 

 like Sagit/ii, manv annelids, and .-iDipliidxiis: there are also animals in 

 which the mesoderm consists of mesenchyme and mesothelium: either the 

 mesenchyme arises iirst and later tlie mesothelium, as in the ecliinoderms, 

 or in the reverse oriler, as in most vertebrates. 



Histological and Organological Differentiation. — All the organs of 

 an animal arise from the three germ-layers. 'Idte iletails differ in the 

 various groups; the following is the most general: front the i\'lodcr»i arise 

 the skin with its glands and appendages, the ner\ous system, and the 

 sensory epithelium; the entoderm gives rise to the most imp)ortant part of 

 the digestive tract with its glands; while muscles, blood, supporting and 

 connective substances, excretory organs, in whole or in part, arise in the 

 ■mesoderm; the sexual organs are also usually mesodermal. 



Relations of the Germ-layers in Budding. — The question has been raised 

 as to how far the germ-layer theory is appHeable to llie occurrences in asexual 

 reproduction. At first one would expect that each ore:an of the daughter would 

 arise from the corresponding orj;an of ihc maternal animal, or at least from a 

 mass of tissue belonging to one of the same germdavers. In manv instances 

 this is tlie case; in the buddiui; of hydroids the eiUodrrm ami ectoderm of the 

 bud arise from the correspondiiig layers of the malei-nal Inuh- (,lig. 03"). but 

 execptions are known. In jiolyzoans and lunicales (here are undilTerenlialed 

 cells which are employed in cases of Inulding. In the reneneration of lost parts 

 it is not necessary lluu die missing structure should be i-e-formed l^v the same 

 layer from which it originally arose. TIk- lens of 'J'riloii arises onlogenelieally 

 from tlie epithelium of tlie skin. If extirpated, it is regenerated from the pig- 

 mented epiitheliuni of llie iris. 



Reviev,? of the Different Kinds of Reproduction.— The fore^'oing outline 

 of rcproduelion is in accordance witli the pre\ailing ideas. Alllunigli those arc 

 justilied iheorelieall)', they do not correspond to the actual relations, since the 



