MARINE TELEOSTEAN DEVELOPMENT. 79 



in the eggs. They present a similar appearance to those of the 

 previous day, though the increasing complexity of the gill- 

 region is evident, and four clefts are visible, extending outwards 

 to the otocysts. 



The heart is larger and the two chambers are more distinct. 



25/i April, Iltk day. Black pigment-spots are sparsely 

 scattered over the dorsal and lateral regions and a few on the 

 tail. 



The eye has a bright bronze hue. 



The remainder of the embryos emerged on this day. 



The subsequent changes are best described under the 

 special head of each organ, so that we have now to retrace 

 our steps and observe what is the history of the various 

 organs we have mentioned. These organs may be classified 

 according to their origin from each of the three primary layers 

 of the germ, namely, the outer layer or epiblast, the middle 

 layer or mesoblast, and the inner layer or hypoblast. They are 

 most conveniently described under these heads. 



ORGANS DERIVED FROM THE EPIBLA.ST OR OUTER LAYER. 



Nervous system. 



We have seen that the outer layer or epiblast was esta- 

 blished as a single layer of cells in the developing disc or 

 blastoderm. These cells early become flattened and in section 

 spindle-shaped. The outer or corneous layer at first covers the 

 trunk of the embryo and the arc of the blastoderm beyond, but 

 later, cellular thickening occurs beneath forming the nervous 

 layer of the epiblast. We have, indeed, seen that this 

 thickening forms the keel (carina) that presses on the yolk. It 

 becomes the rudiment of the spinal cord (neurochord). Its con- 

 stituent cells are full and rounded but, as downward proliferation 

 proceeds, those forming its lateral borders become columnar 

 and unmistakeably mark off the neurochord from the adjacent 

 cells, especially anteriorly. In the latter region they proliferate 

 so rapidly, ventrally and laterally, that they come into contact 

 with the inner layer or hypoblast and thus leave little room for 

 mesoblast. Both layers of epiblast seem to extend over the 



