96 TEXT-BOOK OF EMBRYOLOGY. 



mesoderm outside of the embryonic disk is an intermediate step between the 

 formation of mesoderm entirely within the embryonic disk and its formation 

 around the entire vesicle, as in the hypothetical case. 



Neither in Peters' nor in Graf Spec's ovum is any embryonic body cavity 

 present. But in both cases a very large cavity exists between the mesoderm of 

 the yolk sac and that of the chorion. This cavity the extraembryonic body 

 cavity (exoccelom) probably arises by a splitting of the extraembryonic meso- 

 derm into two layers, parietal and visceral, just as the embryonic body cavity in 

 other Mammals is the result of a splitting of the intraembryonic mesoderm 

 (p. 87). The splitting would occur as shown in Fig. 89, C. The parietal 

 layer which with the trophoderm becomes the chorion, then grows rapidly and 

 becomes widely separated from the visceral layer, the latter with the entoderm 

 constituting the wall of the yolk sac. Thus a stage is reached which is shown in 

 Fig. 89, C, and which corresponds with Peters' ovum (Fig. 83). The embry- 

 onic disk with its yolk sac and amniotic cavity occupies but a small space within 

 the chorionic vesicle. 



The stage corresponding to Graf Spec's ovum would be produced by a fur- 

 ther splitting of the mesoderm in the roof of the amnion, so that finally the em- 

 bryonic disk and yolk sac remain attached to the chorion only by a band of 

 mesoderm, the belly stalk (Fig. 90; compare with Fig. 85). 



Even at this stage, no body cavity is present within the embryonic disk 

 (Fig. 86). When it does appear, however, it becomes continuous laterally with 

 the exoccelom (see Chap. XIV), and the parietal and visceral layers of meso- 

 derm within the embryonic body are continuous, respectively, with the parietal 

 and visceral extraembryonic mesoderm. 



PRACTICAL SUGGESTIONS. 



For a complete demonstration of the formation of the germ layers, especially of the 

 mesoderm, in any class of animals, sections of many successive stages are necessary. A few 

 specimens, however, suffice to illustrate certain principles of development. 



The formation of the gastrula in Invertebrates is fairly well illustrated by the star-fish. 

 After the blastulae appear (see "Practical Suggestions," p. 53), they may be observed from 

 time to time within the next few hours and the various steps in the invagination process 

 made out quite definitely. Any of these stages may be preserved in the same manner as the 

 earlier stages (p. 53). 



The formation of a gastrula from a blastula, in which there is a considerable amount of 

 yolk, is seen in the case of the common frog. The ova are taken at a time when a small 

 crescentic depression (the blastopore) appears at the marginal zone, and at intervals from 

 this time until the yolk plug is seen projecting through the blastopore. They are fixed in 

 5 per cent, formalin, stained in toto in borax-carmin (p. 53), cut in rather thick sections in 

 celloidin, and mounted in xylol-damar. The most instructive sections are those which pass 

 through the blastopore, in the meridional plane. The gelatinous mass which surrounds each 

 egg is coagulated when transferred to alcohol and may then be removed by means of needles. 



