5^0 EMBRYOLOGY 



stalk of the allantois. The amnion is the ' caul ', and the placenta 

 is shed as the ' afterbirth '. 



NOURISHMENT OF YOUNG STAGES 



The resemblance between the development of the minute, 

 yolkless eggs of most mammals and that of the bird's egg, which 

 is large and yolky, is a remarkable fact. It suggests that the 

 ancestors of these mammals had yolky eggs as the Monotremata 

 still have, that they acquired the habit of retaining them within 

 the body, and that there the allantois, which in the bird's egg 

 serves principally for the respiration of the embryo, enabled the 

 mother to make complete provision for the nourishment as well 

 as for the respiration of her offspring, and with that the yolk 

 disappeared. The Chordata whose embryology we have been 

 studying exemplify well the several ways in which animals are 

 nourished during their development. The lancelet obtains its 

 own food as a larva. The frog during its early stages, and the bird 

 throughout development, subsist upon yolk with which the 

 ovum was stocked by the mother. Birds are provided also with 

 nutriment (the ' white ') around the ovum in the shell. Mammals 

 are nourished directly from the mother's body both before and 

 after birth. 



DEVELOPMENT OF INVERTEBRATES 



With the embryology of invertebrates we can only deal very 

 briefly. The development of Hydra and Obelia has already been 

 described (pp. loo and 107). It includes complete and equal 

 cleavage of the ovum, a hollow blastula, the conversion of this 

 into the two layered (gastrula) stage by the immigration of 

 some of the cells of its wall, the origin of the enteron as a spHt 

 in the mass of endoderm, and the shaping of the gastrula into 

 the adult by a simple process. In the starfish cleavage is complete 

 and equal and ends in the estabUshment of a blastula, not unUke 

 that of the lancelet, from which by invagination a gastrula is 

 formed. The mesoderm arises from the anterior end of the arch- 

 enteron, as a pouch whose cavity gives rise to the coelomic spaces. 



In the earthworm (Fig. 525) and swan mussel cleavage of 

 the ovum is complete but unequal, and forms a hollow blastula, 

 which invaginates to give rise to a gastrula. The mesoderm arises 



