entoderm. As the body axis is laid down behind this area, 

 the notochord is separated at the primitive node from the 

 entoderm; laterally and anteriorly, mesodermal plates ex- 

 tend out and later form the somites and enclose the coe- 

 lomic space. The extraembryonic mesoderm in the human 

 appears early by egression of cells from the central mass. 



In the marsupials there is some variation in eggs. That 

 of the opossum has a distinct albumin layer enclosed by a 

 shell membrane. The egg is isolecithal, the cleavage holo- 

 blastic; the resulting cells tend to orient them.selves around 

 an internal blastocoel. 



Among the placentals there may be some albumin out- 

 side the zona pellucida, or chorion. The egg is isolecithal, 

 centrolecithal, or alecithal and cleaves holoblastically to 

 form a solid ball of cells, the morula. From the morula a 

 blastula is formed by the development of a central hollow 

 (Figure 7-15). The development of this hollow also produces 

 a thick mass of cells at one end. From this central mass the 

 embryo and its overlying membranes develop. In some of 

 the marsupials, a central mass of cells is only poorly devel- 

 oped or even lacking. In the latter case, a central mass is 

 produced by increased mitoses at one end of the blastula. 



The outer layer of cells of the placental blastula forms a 

 trophoblast. The central mass gives rise to entoderm by in- 

 gression of cells which spread out as a lining layer, convert- 

 ing a part of the blastocoel to an archenteron. The central 

 mass, inside the trophoblast, now becomes an epiblast. In 

 some placentals the trophoblast overlying the central mass 



disintegrates, whereas in other mammals, the primates, or 

 rat, the trophoblast remains. 



Development from this point follows two lines. In those 

 with the overlying trophoblast, as amniotic cavity, enclosed 

 by epiblast cells, hollows out above the embryonic disc. In 

 the other group, this cavity develops by a sinking of the ger- 

 minal disc and an overgrowth by anterior and posterior folds. 

 In the latter group, the process of amnion formation is like that 

 observed in the chick or the reptile (Figure 9-6). 



In the mammal the process of mesoderm formation in- 

 volves the development of a primitive streak, pit, and node 

 as in the lower forms. It should be noted, however, that this 

 is a mesodermal "gastrulation." In marsupials and more so 

 in the placentals, the trophoblast, which corresponds to the 

 chorion of the reptile, is involved in the placentation of the 

 embryo. 



PLACENTA 



Possession of a placenta, the device by which the foetus 

 receives nourishment from the maternal animal, is not 

 limited to mammals, and it is not always the same. In the 

 shark, for example, the function of the placenta and the re- 

 lationship between the young and the mother varies. In the 

 case of the Spiny Dogfish, it is one of shelter and protection. 

 In other sharks, the uterine lining secretes a nutritional 

 material necessary, or at least helpful, to the development 

 of the voung. The most advanced type involves a yolk-sac 



A 4 CELL 



B THIRD CLEAVAGE 



C BLASTULA 



D SECTION OF BLASTULA 



external gills 



notochord 



liver diverticulum 



E EARLY LARVA 



LATE LARVA 



Figure 7-13. Stages of the lungfish lepidosiren. A, 4-cell stage; B, third cleavage furrow^s; 

 C, external appearance ot early blastula; D, section of early blastula; E, tail-bud larva; F, sagittal 

 section of early larva. (After Kerr) 



208 



EMBRYOGENESIS OF THE CHORDATES 



