178 INTERNAL FACTORS IV. i 



3. PISCES. 



Morgan has shown that in the fishes Ctenolabrus, Serranus, 

 and Fundulus there is no definite relation between the first (or 

 second) furrow and the median plane of the embryo. 



In Fundulus when one of the first two blastomeres is removed 

 the other gives rise to a perfect embryo of more than half-size. 

 In segmentation the first furrow is in the plane of what would 

 be the second furrow of the whole egg, the second in that of 

 the third, and the third more or less in that of the fourth ; but 

 as the successive furrows are at right angles to one another in 

 both cases it is permissible to assert that the segmentation of 

 the half blastomere is total. 



Some of the protoplasm of the half that has been removed 

 flows in underneath the other, is nucleated by it and added to its 

 mesoderm. The size of the nuclei is the same in \ and \ embryos. 



Morgan also found it possible to remove two-thirds of the 

 yolk, but not more, without interfering with the normal develop- 

 ment of the whole egg. 



Injury to the germ-ring on one side of the dorsal lip resulted 

 in a deficiency in the mesoderm of that side posteriorly ; 

 anteriorly both sides were equally well formed. 



So Kastschenko has shown for Elasmobranchs and Kopsch for 

 Teleostei and Elasmobranchs that injury to the lip of the blasto- 

 pore will only interfere with the normal bilaterality of the embryo 

 when effected quite close to the middle line. 



A series of experiments similar to the last has been carried 

 out by Sumner on the eggs of various fish (principally Fnndulus, 

 also Exocoetus, Salvelinus, and Batrachus). 



By means of needles inserted into various parts of the blasto- 

 derm the exact mode in which the material of the latter is used 

 in the formation of the embryo is first determined. It is 

 shown that the material for the embryo is brought into position 

 not by a concrescence of the lateral lips of the blastopore (sides 

 of the germ-ring), but rather by an axial concentration of the 

 cells originally situated in that ring at the posterior margin or 

 dorsal lip, the cells so concentrated being continually pushed 

 forwards in the middle line until the anterior end of the embryo 

 comes to lie near the original centre of the blastoderm. 



