Evolution of Fishes from Invertebrates 93 



will soon be forthcoming. We will here trace mainly the 

 "direct" mode, as being typical of all freshwater, land, 

 and some marine forms. 



The egg undergoes holoblastic and nearly equal seg- 

 mentation inside the egg-case. The cells, pushing apart, 

 leave a large blastocoel cavity and soon all of these cells 

 form cilia. Next by invagination of half of these cells, a 

 two-layered gastrula is formed. In Amphioxus the egg 

 also shows holoblastic and nearly equal segmentation in- 

 side the egg-case, a similar blastocoel cavity is formed, and 

 invagination occurs. Only the outer or epiblast cells here 

 however become ciliate, though areas or patches of internal 

 ciliate cells may later be formed by the embryo. Amongst 

 cyclostome fishes Petromyzon shows holoblastic though 

 decidedly unequal segmentation; and this leads up to the 

 condition in Myxine which in this, as in so many other 

 modified and derivative details, shows a meroblastic type. 

 In the former a restricted kind of invagination takes place, 

 giving rise to a small-celled epiblast and a larger-celled 

 hypoblast, as is true also of Amphioxus. Cilia, however, 

 are absent and only later form on restricted areas of epi- 

 blast or hypoblast. In higher fishes either holoblastic or 

 meroblastic segmentation is seen, according to the group 

 studied, and variations also are seen in formation of epi- 

 and hypo-blast. 



The embryo, when freed from the egg-case in all of 

 the above, soon assumes a somewhat elongate shape, and 

 in succession the neural ridges that unite to form the 

 neural canal, also the various anterior sense organs, appear 

 in all in fundamentally similar and successive manner. 

 Meanwhile a few cells, set free into the area between epi- 

 and hypo-blast, give rise to five patches or plates of cells 

 in nemerteans and in Amphioxus ; or two rather irregular 

 plates in Petromyzon and other fishes, which apparently 

 represent a condensation and fusion into pairs between 

 four of the above five. These are the first mesoblastic 

 rudiments of the coelomic plates. Alike in nemerteans and 

 in Amphioxus one plate is anterior, two are antero-lateral 

 and two are medio-posterior. In nemerteans by hollowing 

 out and continued growth of the posterior pair, the alimen- 



