Semon's view 



Formation of the Germ Layers in an Ophiurid. 1 — Amphiuea 



squamata (Say) had been incompletely studied in 1870 by Metschnikoff 

 who argued from analogy that there was here an invaginate gastrula 

 though he did actually observe it. In 1882 Apostolides claimed that 

 it was formed by delamination, but he gave no figures. Fewkes in 

 1887 confirmed this view but did not add much to its substantiation 

 owing to lack of material. 



In the present paper Russo considers the formation of the blastula, 

 the entoderm and the mesoderm in this species. 



Cleavage gives rise to a very characteristic blastula having elongated 

 cells surrounding a rather small cleavage cavity. 



The inner part of each cell is made very opaque by a quantity of 

 pigment which is thought to be related to the presence of food-yolk: 

 the outer part of each cell is yellow and transparent. The blastula 

 thus looks like that of Geryonia. 



The inner pigmented part of each cell becomes divided from the 

 outer part and the resulting gastrula has an ectoderm of transparent, 

 elongated cells with large nuclei and an entoderm of smaller, rounded, 

 pigmented cells. 



He thus extends the occurrence of delamination to a new group. 

 Following Brouer he attributes this mode of gastrulation to the condi- 

 tions of development. " Every where when a free-swimmiug blastula 

 is present we find unipolar entoderm formation, corresponding to tbe 

 direction of swimming : everywhere when development goes on in a 

 limited space we find multipolarity." This the author thinks is true 

 of the Echinoderms as well as of the Coelenterates. 



The proctodoeum and the archenteron are formed by the breaking 

 down at a definite point, first of the ectoderm and then of the ento- 

 derm. After this the mesoderm appears : first as a heap 

 shaped cells on either side of the proctodoeum. These ■ 

 delamination from the ectoderm and, pushing backward, nearly fill* e 

 whole cleavage cavity. They finally arrange themselves upon the 

 ectoderm and the entoderm, like an epithelium, thus forming a sing e 

 cavity, the coelom.— G. \V. Field. 



The Origin of the Sertoli's Cell.— It is now generally known 

 that the seminiferous tubules of a mammalian testis contain twofunda- 



*AcW!le Russo: Zool. Anz. Nov. 16, 1891. 



