206 PEOP. HTJXLET ON THE 



The couversion of the Morula into the Gastrula may take place in 

 several ways. 



In the simplest, the Morula, being composed of equal or nearly 

 equal blastomeres, these, undergoing conversion into cells, differ- 

 entiate themselves into an epiblast, which invests the remaining 

 cells, constituting the hypoblast. The central cells of the hypo- 

 blast next diverge and leave a space filled with fluid, the alimen- 

 tary cavity, which opens at one end, and thus gives rise to the 

 Gastrula. This is the process generally observed in Porifera, 

 Coslenterata, Turbellaria, Trematoda, and Neraatoidea. 



In a second class of cases, the Morula becomes converted into 

 blastomeres of unequal sizes, a small and a large set. The smaller 

 are rapidly metamorphosed into cells, and invest the larger(with any 

 remains of the food-yelk) as a blastoderm. The hypoblast arises 

 either from the blastoderm thus formed, or from the subjacent 

 larger blastomeres. This is the process observed in certain Tur- 

 bellaria, in the Ctenophora, in most of the Oligochaeta and Hiru- 

 dinea, in the Arthropoda, and in most Vertebrata. 



In a third group of instances, the Morula, whether consisting 

 of equal or unequal blastomeres, becomes spheroidal, and encloses 

 a correspondingly shaped blastocoele. One part of the wall of this 

 vesicular Morula then becomes invaginated, and is converted into 

 the hypoblast, which encloses the alimentary cavity, the latter com- 

 municating with the exterior by the aperture of invagination. 

 This process has been observed in the Chsetognatha, Echinoder- 

 mata, and some Glephyrea, in Lumbricus and Hirudo — in poly- 

 chsetous Annelida, Enteropneusta, Brachiopoda, and most Mol- 

 lusca — and in AmpJdoxus, Petromyzon, and the Amphibia among 

 the Vertebrata. 



The various modes in which the two primary layers of the germ 

 may be developed shade off into one another, and do not afi"ect 

 the essence of the process, which is the segregation of one set of 

 cells to form the external covering of the body, and of another to 

 constitute the lining of the alimentary canal. We may, with 

 Haeckel, term those animals which pass through the Grastrula 

 stage, GastrecB. The Gastrula may be deeply cup-shaped, or flat- 

 tened out into a disk, slightly concave on one side ; but in what- 

 ever manner the Gastrula is formed, and whatever be its shape 

 when its alimentary cavity is complete, one of two things hap- 

 pens to it. It becomes provided with many ingestive apertures 

 distinct from that first fox*med (polystomatous), or with one only, 



