204 



PRINCIPLES OF ANIMAL BIOLOGY 



In insects the germ cells usually either are larger (Fig. 175) or contain 

 certain granules not found in somatic cells. In vertebrate animals the 

 distinction between somatic and germ cells is not recognizable until a 

 much later stage. In the embryos of a number of forms the germ cells 

 are found as large cells in the lining of the digestive tract (Fig. 176), 

 whence they migrate up through the mesentery and out to the place 



where the gonads subsequently develop. 

 Whether germ and somatic cells have 

 existed as distinct entities through the 

 earlier embryonic stages is not known. 



Gastrulation. — When the blastula is 

 well formed, it is converted into a two- 

 layered embryo. The process by which 

 this conversion is effected, already briefly 

 outlined in C'hap. G, is called gastrulation. 

 The simplest form of invagination takes 

 place in those animals whose eggs have a 

 small amount of yolk evenly distributed, 

 that is, in alecithal or homolecithal eggs. 

 In these the vegetative side of the 

 blastula becomes flattened, then in- 

 turned (invaginated) (Fig. 177, above). 

 The invagination proceeds until the 

 inturned cells are in contact with the 

 opposite side of the blastula wall. The 

 embryo now has two layers of cells, an 

 outer or ectoderm and an inner or etido- 

 dcrm. The blastocoele has been obliter- 

 ated, but a new cavity, the archcntcron, 

 lies within the endoderm. This cavity 

 has been pushed in from the outside, 

 with which it is still connected by a 

 small opening called the blastopore. The two-layered embryo of this 

 stage is known as a gastrula. The endoderm of the gastrula becomes 

 the lining layer of cells of the digestive tract of the adult. 



The blastula produced from a mildly telolecithal egg could not well 

 be invaginated directly from the vegetative side by flattening and 

 infolding, because the layer of cells there is so thick. In such a blastula 

 the invagination begins about midway between the ar\imal and vegetative 

 poles, where the cell layei- is thinner (Fig. 177, below). It is mostly the 

 cells above the blastopore which are invaginated, though there is some 

 withdrawal of the whole yolk-laden mass of lower cells into the interior. 

 The end result is, as in the alecithal embryo, a two-layered gastrula. 



Fig. 175. — Early recognition of 

 germ cells (gc) in the development of 

 the egg of the fly Miastor, showing 

 also the cleavage cells (cl) at the 

 periphery and the yolk globules (y). 

 (After Hegner in Journal of Alor- 

 phology.) 



