494 KEPORT OF COMMISSIONER OF FISH AND FISHERIES. [40] 



from A to B. This figure also shows the very narrow slit-like lumen of 

 the segmentation cavity to the right of A, and its extent over the left 

 half of the blastoderm is showu by the line below and to the left of sg, 



9.— The development of the germinal layers. 



This portion of the subject is one upou which I cannot, unfortunately, 

 throw much light on the basis of observations made upon the develop- 

 ment of the cod's egg, and I shall therefore place under contribution 

 the labors of CEUacher and others on the trout, and my own observa- 

 tions upou those of several other species;. It is evident that in respect 

 to the developmental changes which the blastoderm undergoes in dif- 

 ferent species, there is considerable variation. In the trout, for example; 

 the embrj^ouic shield or area, corresponding in the cod's egg to the space 

 from A to B in Figs. 16, 17, and 18; there is a considerably less promi- 

 nent development of the embryonic shield at a correspondingly early 

 stage. In Tylosurus, at an early stage, the conditions of the two in re- 

 spect to the size of the embryonic shield is about the same. 



The embryonic shield, as development advances, grows farther and 

 farther inwards towards the center of the blastodermic disk, or rather, 

 as it grows in length before and behind, the disk at the same time spreads 

 in consequence of the continued segmentation of its component cells, so 

 that these are spreading themselves over a greater and greater area 

 while they are at the same time undergoing a definite rearrangement 

 into strata or layers, each of which has a definite share in building up 

 the different parts of the embryo's body. This mode of spreading, how- 

 ever, never affects the relation of the embryo to the edge of the disk. 

 Its tail-end lies at the edge, its head at the center for a considerable 

 time (in small or moderate sized ova constantly), with the axis of the 

 body of the future embryo lying in one of the radii of the disk. In un- 

 usually large ova, like those of the salmon, Tylosurus and Arins, the 

 blastoderm spreads so fast after a while that the embryo does not grow 

 in length rapidly enough to maintain the position of the head near the 

 center of the blastoderm. These last facts explain CEUacher's position 

 in regard to this phenomenon in the trout's ovum. 



With the development of the embryonic shield the diflierentiation of 

 the lower layers commences. The first to be split off is the sensory or 

 epiblastic layer; in the cod's egg this is formed on the seventh day in 

 ova which hatched in sixteen days. The process is truly one of delami- 

 nation, and cannot be regarded as i)roduced by a true gastrulation at 

 all, as Haeckel has tried to show in a paper already noticed. The pro- 

 cess of the differentiation of the layers we saw began with the develop- 

 ment of an epithelial layer of epiblast over the surface of the germinal 

 disk before the appearance of the segmentation cavity. Immediately 

 after the appearance of the latter the embryonic disk begins to be de- 

 veloped and the layers differentiated. The sensory layer is split off 

 first from the underlying sti^atum of cells at the head end of the embry- 



