STAGES B TO G. MEDULLARY GROOVE. 299 



becoming much thickened, and its cells two or three deep in the 

 anterior parts of the embryo. (PI. 10, fig. 2.) 



In the succeeding stages that part of the epiblast, which will 

 form the spinal cord, gradually becomes two or three cells deep. 

 This change is effected by a decrease in the length of the cells 

 as compared with the thickness of the layer. In the earlier 

 stages the cells are wedge-shaped with an alternate arrange- 

 ment, so that a decrement in the length of the cells at once 

 causes the epiblast to be composed of two rows of interlocking 

 cells. 



The lateral parts of the epiblast which form the epidermis of 

 the embryo are modified in quite a different manner to the 

 nervous parts of the layer, becoming very much diminished in 

 thickness and composed of a single row of flattened cells. 

 (PL 10, fig. 3.) 



Till the end of stage F, the epiblast cells and indeed all the 

 cells of the blastoderm retain their yolk-spherules, but the epi- 

 blast begins to lose them and consequently to become transparent 

 in stage G. 



Medullary Groove. 



During stage B the medullary groove is shallow posteriorly, 

 deeper in the middle part, and flattened out again at the extreme 

 anterior end of the embryo. (PI. 7, fig. 10 a, b, c.) 



A similar condition obtains in the stage between B and C, 

 but the canal has now in part become deeper. Anteriorly no 

 trace of it is to be seen. In stage C it exhibits the same general 

 features. (PI. 10, fig. 2 a, 2 b, 2 c.) 



By stage D we find important modifications of the canal. 



It is still shallow behind and deep in the dorsal region, PI. 

 10, figs. 3^, 3*?, 3/; but the anterior flattened area in the last 

 stage has grown into a round flat plate which may be called the 

 cephalic plate, PI. 8, D and PI. IO, figs. 3 a, 3 b, 3 c. This plate 

 becomes converted into the brain. Its size and form give it 

 a peculiar appearance, but the most remarkable feature about it 

 is the ventral curvature of its edges. Its edges do not, as might 

 be expected, bend dorsalwards towards each other, but become 

 sharply bent in a ventral direction. This feature is for the first 



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