426 SUMMARY OF CURRENT RESEARCHES RELATING TO 



not central as it is in Amphioxus, and by the overgrowth of the 

 smaller elements ; the first part which now becomes evident is the 

 head, which is formed by a shallow depression of the ectoderm just 

 below the blind end of the archenteric cavity. The layers of the 

 upper half of the egg are formed by invagination and proceed from 

 behind forwards. The ectoderm and endoderm lie close to one 

 another in the dorsal median line, and on either side of this there 

 appear large irregular aggregations of cells, which are intercalated 

 between the ectoderm and endoderm ; the mesoderm, therefore, so far 

 as its dorsal and larger portion is concerned, also arises by a process 

 of invagination, and it has just the same relations as in the Selachii 

 and the Triton. The mesodermal cells increase very rapidly in the 

 dorsal lip of the blastopore and form a continuous plate in the middle 

 line. Shortly after this the germ-layers are developed in the ventral 

 and lateral regions, by the growth of the smaller blastomeres ; the 

 mesoderm of this part owes its origin to a differentiation of the yolk- 

 cells ; but, as the yolk-cells, in this creature belong to the endoderm, 

 it follows that in Petromyzon, at any rate, the ventral mesoderm is 

 formed from the endoderm. 



At about this period the ectoderm consists of cylindrical epithe- 

 lium-cells, higher and more delicate in the dorsal median line ; the 

 cells of the invaginated endoderm are likewise cylindrical, but are 

 generally larger. At this period, too, the cells are all fitted with 

 yolk-granules, so that it is difficult to see their boundaries ; this 

 arrangement persists until the latest stages, when many parts of the 

 organism are completely free from granules. As the author points 

 out, the mode of formation of the germ-layers is very similar to what 

 obtains in the Triton, and can, without difficulty, be brought into 

 agreement with the history of the same parts in the Selachii. 



After a discussion and comparison of his results with those 

 obtained by other observers on allied animals, Mr. Scott passes to 

 the endoderm. Commencing with the chorda dorsalis, he shows 

 that, at the conclusion of the invagination process, the embryo 

 is bilaminate in its dorsal middle line, and trilaminate at the 

 sides ; the only exception to this arrangement being the upper lip of 

 the blastopore into which the mesoderm is continued. There is a 

 single layer of ectodermal and a similar single layer of somewhat 

 larger endodermal cells in the middle line ; soon there appears the 

 first rudiment of the dorsal medulla, and, at the same time, there is 

 seen a layer of delicate cells which separates the endodermal cells of 

 either side from a median aggregation of cells. This last is the fii'st 

 rudiment of the notochord ; the constituent cells become higher and 

 narrower, and get separated off from the archenteric cavity by the 

 down-growth of the adjacent endodermal cells. Very rapidly there 

 appears a prismatic solid cord consisting of a single layer of cells, 

 extending through the whole length of the body, and distinctly marked 

 off. The chordal cells next divide and give rise to a number of 

 irregular smaller multi-angulate cells, which take on a radial arrange- 

 ment. Soon the cells lose their yolk and become vacuolated ; the 

 anterior end now becomes bent down at right angles. The author 



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