156 PRINCIPLES OF EMBRYOLOGY 



Different adherents of this view, however, have very different ideas as 

 to the nature of the active process by which the endoderm originates. 

 Perhaps the simplest of these is that of Patterson (1909), who supposed 

 that the posterior edge of the cellular plate becomes folded under and 

 grows forward below the remaining part. This has become one of the 

 commonest accounts given in textbooks, perhaps because of its apparent 

 simphcity; but, unfortunately, the evidence for it is negligible, and all 

 authors who have examined the matter for the last forty years (except 

 Lutz 1953, 1954) have denied it. Another, more plausible, view is that 

 isolated cells are pushed out from the cellular plate and gradually build 

 up a lower layer, Jacobson (1938) believes that endoderm formation 

 begins in this way, but that the process goes on fastest in the posterior 

 region (though not quite at the posterior margin) and eventually attains 

 such an impetus there that the whole plate is bent down into a groove 

 and migrates en masse into the endoderm. He describes the formation of a 

 centre of invagination which would really merit the name of a blastopore 

 and closely resembles the structures which, we shall see, are undoubtedly 

 formed at a much later stage when the invagination of mesoderm occurs. 

 Most later authors have been unable to confirm Jacobson's account in full. 

 A final view (Hunt 1937) which must be mentioned is that, however the 

 lower layer of cells is produced at this stage, it eventually does not form the 

 endodermal organs of the embryo, such as the gut, but is at a later stage 

 pushed out to the sides by cells which come out of the upper layer (from 

 the primitive streak, which forms there, see later). 



Although it is not possible to decide fmally between these possibilities 

 at the present time, the safest view is probably that the greater part of the 

 endoderm is formed in the first way mentioned, by a splitting or delamina- 

 tion of the original cell mass. On this interpretation the blastocoel is not 

 represented by the subgerminal cavity, but by the cleft which separates 

 the upper from the lower layer. 



During the period when the lower layer is forming, the mass of cells is 

 also becoming thinner and spreading more widely over the surface of the 

 yolk. From tliis time on it is usually knowoi as a blastoderm or blastodisc, 

 and its upper and lower layers are frequently — and non-commitally, a useful 

 point in the circumstances — referred to as the epiblast and hypoblast 

 respectively. Moreover, a certain differentiation is appearing in plan, 

 hi the more central part, the cells are beginning to have used up their 

 content of yolk granules, so as to become more transparent, while all 

 round the periphery they remain heavily charged and opaque. As the 

 subgerminal cavity becomes more defmite below the central region, this 

 differentiation into two concentric zones increases until there is a well- 



