270 OUTLINES OF EVOLUTIONARY BIOLOGY 



(Fig. 119, IX) is very much disguised in the ontogeny, simply 

 owing to the presence of a large quantity of nutrient material 

 in the egg. 



In the development of the bird's egg (and also in that of reptiles, 

 which is closely similar) it is still more difficult to recognize the 

 process of gastrulation. The quantity of yolk (Fig. 70) is so 

 enormous that it entirely prevents cell-division from taking 

 place in the greater part of the egg, and segmentation is con- 

 sequently confined to a small yolk-free area at one pole of the 

 spherical mass, where it gives rise to a layer of cells known as the 

 blastoderm. This blastoderm gradually spreads over the yolk and 

 from it the embryo is formed by a kind of pinching-off process, 

 the yolk remaining enclosed in a bag the yolk-sac attached to 

 the lower surface of the embryo, until it is all absorbed by means 

 of a special set of blood-vessels developed in the yolk-sac, and 

 used up in the nourishment of the growing embryo. The 

 embryo itself meanwhile becomes provided with special organs 

 for its protection and respiration, the so-called foetal membranes. 

 Of these the amnion, formed by outgrowth of the embryonic 

 body-wall, forms a sac filled with fluid, which surrounds the 

 embryo at a distance and leaves it free room for growth, while 

 the allantois, formed by outgrowth of the embryonic gut-wall, 

 forms a highly vascular organ which comes into relation with 

 the extremely porous shell and thus provides for the necessary 

 gaseous interchange. 



The yolk-sac, the amnion and the allantois are all organs 

 which have been developed in accordance with the special 

 requirements of the embryo, and which must therefore be left 

 out of account in reconstructing the direct ontogenetic record. 



In the dogfish also (Fig. 120) we find an excellent illustration 

 of the formation of a special yolk-sac to contain the enormous 

 supply of food material for the developing embryo, but in this 

 case neither amnion nor allantois is developed. 



The Mammalia, on the other hand, have adopted an improved 

 method of nourishing their young. The developing embryos are 

 retained within the body of the parent and obtain their food 

 supply from the blood of the latter by means of a special organ 

 known as the placenta, which is developed partly from the 

 allantois of the embryo or " foetus," and by which the latter is 

 attached to the wall of the uterus. This method seems to have 

 proved much more satisfactory than the provisioning of the egg 



