334 



GENERAL ORNITHOLOGY 



PART II 



(gastrula), is common to all animals which consist of more than 

 single cells, under various modifications and disguises ; the process 

 described is that occurring in meroblastic eggs which have a discoidal 

 cleavage and form a discogastrula.^ 



What we have got now is a tread or germ consisting of a cir- 

 cular concavo-convex disc of two layers of blastoderm, resting by 

 its rim upon the great yellow ball of food-yelk, from which it is 

 separated by a cavity, as a watch-crystal from its face. All these 

 changes, up to completion of gastrulation, may go on hefme the egg is 

 laid, the tread of a perfectly fresh egg being already a multicellular 



Fig. 112.— Further development of hen's egg; after Haeckel: A, the mulberry mass of 

 cleavage cells, b, same as seen on top in Fig. Ill, F, here viewed in profile in section, resting 

 upon 71, the simply-shaded part of the figure, to represent conventionally the mass of food- 

 yelk. A, morula stage (as before) ; B, blastula stage, the mass of cells, b, forming the blasto- 

 derm, uplifted from the food-yelk, leaving the cleavage-cavity, s ; lu, the thickened rim of the 

 germ-disc ; C, the blastula in process of inversion, by which a layer of endoderm-cells, i, grow- 

 ing from periphery to centre, will apply itself to the layer of exoderm-oells, e, obliterating 

 the cleavage-cavity, s; D, the discogastrula completed, by union ofendoderm, i, with exo- 

 derm, e, leaving the primitive intestinal cavity, d, which is quite similar in appearance to 

 the cleavage cavity, s, but morphologically quite different. 



discogastrula. Since the earlier stages of the embryo (cytula, morula, 

 blastula, and gastrula) are actually accomplished while the egg is 

 still in the body of the parent, the analogy of the oviduct to uterus, 

 etc., as well as its strict homology to the parts of a Miillerian duct 

 so named, is not so fanciful as some appear to think. The outer 

 of the two blastodermic layers is the ectoderm or epiUast, C or D, e ; 

 the inner is the endoderm or hypoUast, i. By multiplication of cells 

 between the two arises the mesoUast. The mesoblastic layer of 

 cells subsequently splits into two, of which the outer is the somato- 

 pleura, or body layer, the inner the splanchnopleura or visceral layer. 

 The two-layered germ has then become four-layered. Up to the 



1 The so-called " germ-vesicle " of tlie holoblastic mammalian egg is subsequent 

 to gastrulation, not prior, and is therefore not a blastula proper. 



