74 EDWARD MCCRADY, JR. 



In stage 24 its relation to the endoderm has not changed, but it 

 is no longer a broad plate except at its anterior end where it 

 merges into the prechordal plate, and its posterior end where 

 it lifts above the endoderm and merges into Hensen's node. In 

 early specimens of stage 24 it is throughout most of its middle 

 portion only one or two cells in thickness, the cells being tall, 

 columnar, and extending from the endoderm to the medullary 

 plate. In the totomount (fig. 20) its width at various levels 

 can readily be observed, the narrowest points in this specimen 

 (17176) being at the levels of the hind-brain and again be- 

 tween the sixth and seventh segments. This extreme at- 

 tenuation, however, is peculiar to early specimens. In later 

 ones like 17166 and 17138 the notochord is again a thick plate 

 in the endoderm. 



A mesodermal condensation at the level F in figure 21 is the 

 earliest primordium of the forelimb. I know of no other case 

 in which the forelimb is begun at such an early stage. 



Comparative notes. The stage 24 opossum embryo resembles 

 a rabbit of the same age more than it does any other embryo. 

 It' is nearly identical with Minot's ( '05) rabbit embryo no. 624 

 in the stage of development of the heart tubes, the aortae, the 

 area vasculosa, the notochord, and the proamnion. It is 

 further advanced in having* more somites, having optic and 

 otic anlagen, a forelimb blastema, branchial pouches, infun- 

 dibular groove in contact with hypophyseal plate, and a 

 pharyngeal membrane. It is less advanced in having no 

 caudal amniotic fold. In this last respect it is intermediate 

 between the mammalian type and the reptilian type as seen 

 in Lacerta agilis (Keibel, '04). The early and rapid develop- 

 ment of the first somites is, as will be seen in later chapters, 

 to some extent peculiar to the opossum itself. 



The chick of 4 to 6 somites (23 to 24 hours incubation) is 

 similar in most respects, but has fewer somites and a medul- 

 lary plate which is more advanced with respect to folding, 

 though not with respect to differentiation. 



