92 STRUCTURE OF THE VERTEBRATES 



ing apparatus, and the increasing demand for oil from the thick 

 layer of subcutaneous fat. Ambergris, a cholesterol substance 

 used in the manufacture of perfume, is a secretion of the intes- 

 tinal tract. A great deal of mythology has grown up around the 

 whale. The ''blowing" of a whale is the condensation of the 

 breath in cold air, not a jet of water as usually depicted in 

 story books ; and the belief that it is a fish goes back to biblical 

 times. Even as late as the nineteenth century natural histories 

 called it a fish, a great convenience, as the flesh of the porpoise 

 was a royal dish and with this classification could be eaten 

 during Lent. 



Embryology of the Mammals. The Monotremes, with their 

 large yolked eggs, have developmental processes essentially like 

 those of reptiles. The two higher groups, however, have lost the 

 large amount of yolk in the egg, and specializations of funda- 

 mental importance have developed. The following description of 

 early embryology applies generally in its essentials, but each 

 order has its own modifications which come properly in a study 

 of comparative embryology. The later development of marsu- 

 pials has been mentioned, and the present discussion is limited 

 to the placental, using the rabbit as a type. 



The minute egg is fertilized in the "oviduct", the upper end 

 of the uterine tube, and passes into the uterus. It soon becomes 

 attached to the uterine wall. The early cleavages are holoblastic, 

 and the cleaving ovum forms a hollow, elliptical organism with 

 a small knot of cells at the dorsal pole. This hollow sphere is 

 not a blastula, but the placental homologue of the yolk sac, the 

 knot of cells at the top being the equivalent of the embryonic 

 disc. The embryo, having formed itS' own yolk sac, proceeds to 

 develop in a typically amniote manner. 



The dorsal embryonic cells, or embryonic knot, begins divi- 

 sion and forms a flat plate of cells, the true blastula of the 

 placental mammal. Gastrulation is by an invagination near one 

 edge of the blastula, much like that of the reptile. The amniotic 

 folds grow over the embryo, the inner layer forming the amnion 

 enclosing the amniotic cavity; and the outer layer, encircling 

 the amnion and yolk sac, is the chorion. The latter is in con- 

 tact with the wall of the uterus. The allantois develops as an 



