MBRYOLOGISTS interested in the chick, like everyone else in the E field, find an expanding technology opening up before them. In the last decade, even during the war, microchemical, histochemical, and cytochemical tools have emerged and have developed to a degree that makes it immediately possible to obtain accurate knowledge of even the earliest events in development. It is imperative for embryologists to make certain of the definition of their niorphogenetic problems, in order that the new tools be applied precisely and profitably. The present opportunity to examine the factual basis of some classical concepts as applied to the chick is thus very welcome.Germinal Movements d u h g Gastrulation. The discussion will center around problems of the nervous system, commencing with the relationships of the medullary area to the underlying axial mesoderm. It is characteristic of our uncertainty of the course of early embryonic movements in the chick that Dr. Spratt of Johns Hopkins should have been able, just recently, to niake us readjust all our ideas of early localization by his marking experiments on the unincubated blastoderm.It will be well to recall that gastrulation in t h e chick is performed in a t least two steps, perhaps three. The first, which lias progressed to a variable degree at the time of laying, is the delamination of the lower layer or hypoblast, from an originally single blastodernial sheet. This step takes place without much superficial morphological indication. The hypoblast customarily is believed to give rise t o the entoderm. The mesoderm is invaginated from the upper layer mainly by migration through a morphologically patent blastopore, the priiaitive streak, which takes shape in correlation with other visible changes in the enibryonic tirea. Descriptions are available of some mesoderm, in early stages, rather delaminating irregularly from the upper layer than invaginating in orderly fashion through the streak. The first appearance of the streak itself presents this picture, which might be considered a preliminary or accessory manner of mesoderm formation.In view of the current interest in the association of carbohydrate metabolism with invagination, it is surprising that Jacobson (1938) has thus far been the only investigator t o describe the invagination or delamination process in the chick as being accompnnied by loss of glycogen
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