148 GENERAL PRINCIPLES OF ZOOLOGY 



blastopore (A), thus partially separating a pair of lateral chambers from 

 the rest. The process continues; the blastopore closes, while the 

 entodermal folds extend to the opposite side, where they fuse with 

 the walls (B). In this way a pair of ccvlomic pouches are cut off 

 from the rest of the archenteron which forms the lumen of the 

 digestive tract and its derivatives, while the walls of the pouches 

 form the mesothelium, that of the digestive region the secondary 

 entoderm. In each ccelomic pouch two walls are recognizable, an 

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

 wall of the digestive tract, the splanchnopleure, while the somatic layer 

 unites similarly with ectoderm to form an outer body wall, the soma- 

 topleure. 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 closely connected with the formation of the body cavity. 



Occurrence of Mesenchyme and Mesothelium. There are purely 

 mesenchymatous animals, like the flat-worms, and purely mesothelial, 

 like Sagitta, many annelids, and Amphioocus; there are also animals in 

 which the mesoderm consists of mesenchyme and mesothelium: either the 

 mesenchyme arises first and later the mesothelium, as in the echinoderms, 

 or in the reverse order, as in most vertebrates. 



Histological and Organological Differentiation. All the organs of 

 an animal arise from the three germ-layers. The details differ in the 

 various groups; the following is the most general: from the ectoderm arise 

 the skin with its glands and appendages, the nervous system, and the 

 sensory epithelium; the entoderm gives rise to the most important 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 applicable to the occurrences in asexual 

 reproduction. At first one would expect that each organ of the daughter would 

 arise from the corresponding organ of the maternal animal, or at least from a 

 mass of tissue belonging to one of the same germ-layers. In many instances 

 this is the case; in the budding of hydroids the entoderm and ectoderm of the 

 bud arise from the corresponding layers of the maternarbody (fig. 93). But 

 exceptions are known. In polyzoans and tunicates there are undifferentiated 

 cells which are employed in cases of budding. In the regeneration of lost parts 

 it is not necessary that the missing structure should be re-formed by the same 

 layer from which it originally arose. The lens of Triton arises ontogenetically 

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

 mented epithelium of the iris. 



Review of the Different Kinds of Reproduction. The foregoing outline 

 of reproduction is in accordance with the prevailing ideas. Although these are 

 justified theoretically, they do not correspond to the actual relations, since the 



