84 The Microscope. 



of freedom, where it can obtain the food necessary for its 



growth. 



In other forms, as the Holothurians, 

 another feature appears. As shown by the 

 cut, the pouches are found much as in Am- 

 phioxus, but besides there is an inwander- 

 ing of amoeboid cells, and finally we have 

 eggs in which the mesoblast wholly arises 

 by this method. These points have been 

 fully elaborated by the brothers Hertwig 



F iothurian? tr a a amffibof(i in their valuable paper on the " Ccelom- 



cel Is; c, ccelomatic pouch- . . „ , , . ,, , 



es; h. hypoblast; m. me- theone," but lor our purposes we need 



soblast; s, spicules. , . , ., . , . 



not enter into the various speculations 

 connected therewith and the arguments pro and con. 



In the case of Amphioxus, as illustrated above, the meso- 

 blastic pouches contain a portion of primitive stomach or arch- 

 enteron. After their separation from the hypoblast these spaces 

 become, as we have said, the body cavity. In other forms, 

 where the mesoblast grows in as a solid mass, the body cavity 

 or ccelom appears by a splitting. The mesoblastic pouches con- 

 tinue to grow until finally the segmentation cavity is entirely 

 obliterated. The outer part of the mesoblast (known as the 

 somatoplure) unites with the epiblast, and the inner (the 

 splanchnoplure) with the hypoblast. 



Now, having studied the regular mode of formation of the 

 germ layers, it may be well to say a word as to the parts to 

 which each gives rise. The outer layer, the epiblast, forms the 

 outer layer of the skin, the nervous system, almost all parts of 

 the organs of sense, the various dermal glands, part of the 

 teeth, and the anterior and posterior parts of the digestive tract. 

 The hypoblast gives rise to the epithelium of the middle part 

 of the alimentary canal, the lining of the lungs, air bladder, 

 liver, and other digestive glands, and to the notochord of verte- 

 brates. The mesoblast plays a more extensive role, if not so 

 important an one. From it arise the muscles, genital organs, 

 circulatory apparatus, blood, internal skeleton, urinary organs, 

 dermal bones, and the like. It would be interesting to follow 

 out these various organs in their development in various parts 

 of the animal kingdom, but space does not permit. We must 



