130 A HUMAN EMBRYO BEFORE THE APPEARANCE OF THE MYOTOMES. 



IV. GENERAL DISCUSSION. 



The embryo which we have just described represents an extremely interesting 

 and instructive stage in the ontogenesis of man. In it are found as many important 

 features of early development as could well be expected in one and the same speci- 

 men. Besides presenting so may typical and classical features, it has the added 

 advantage of showing them on an unusually large scale. This size, as already 

 mentioned, may be considered simply as a variation; accentuated it may be by 

 unknown influences. It is well known that certain developmental stages are quite 

 ephemeral ; that there is further a greatly varying susceptibility in different tissues 

 and in these at different times, and herein may lie some explanation of the conditions 

 described above, perhaps an unusual development or late persistence from unknown 

 causes. We may recall here that Rabl makes repeated mention of considerable 

 variations in size, age, and development in the area? embryonales of rabbits, often 

 insisting that they can not be looked upon as either abnormal or distorted, although 

 offering no explanations. We may quote in this connection his own words (I. c., 

 p. 378; cf. also Taf. iv) regarding embryos with one somite: "Da habe ich denn 

 von einer sehr merkwiirdigen Erscheinung zu berichten. Ich habe namlich zwei 

 Arten von Embryonen dieser Entwicklunsstufe beobachtet: die eine war kurz, 

 breit und gedrungen, die andcrc lang, schmal und schlank." The gist of the above 

 is that we consider our embryo normal, though not typical. 



Any discussion of the findings in this embryo naturally revolves around the 

 question of gastrulation and the formation of the germ-layers. We shall not at 

 this time attempt an extended treatment of the subject, but give simply our own 

 interpretation of what we have observed in this particular case. Naturally one 

 should not conclude too much from a single stage, either as to antecedent or later 

 conditions; but every stage must be in harmony with those which precede or follow, 

 and the truth is not always commensurate with the extensivenss of the evidence. 

 On many problems of development this embryo of course throws no light what- 

 soever, being far too advanced. 



As regards the formation of the amniotic cavity and the yolk-sac, we may accept 

 them as currently given. The question of the mesoderm is not so easily disposed of. 

 In spite of its precocious development, we can not yet see the necessity of denying 

 that it may still be, in principle, peristomal mesoderm. Considering the recent 

 attempts of Rabl in this respect and the similar difficulty regarding the entoderm, 

 it would seem to us that the inherent questions of gastrulation and homology 

 should be more definitely disproven before an entirely new and foreign mode of 

 development is invoked. 



In the primitive streak we have a closed blastopore, howbeit radically altered. 

 At its anterior end is an opening and what is theoretically at least an invagination, 

 the head process with its archenteric canal. The posterior end of the streak is, 

 in this stage, marked by the cloacal membrane which is later also open, at present in 

 process of formation. Between the two points there is extensive mesoderm forma- 

 tion, as witnessed by the mitoses in this region, peristomal mesoderm. If one were 



