3887.] 397 [Hyatt. 



specialized and concentrated modes of development may be re- 

 ferred as derivatives. The stages of holoblastic ova may be in a 

 general way classified as follows, to accord with that given above 

 for the Animal Kingdom : 



(1) The ovum or Monoplast (Lankester) ; (2) the first stage of 

 segmentation, which normally results in the production of two 

 cells in the same plane originated by vertical fission, the Mono- 

 placula ; (3) the second stage of segmentation in which two lay- 

 ers arise, the Diploplacula. The first two stages alone seem to 

 have parallel or representative adult forms among Protozoa. The 

 differentiation into esoteric (primitive ectoblast) and enteric (prim- 

 itive endoblast) cells takes place in the Diploplacula, and the mor- 

 phological equivalent of this stage of the ovum, having an upper 

 layer of differentiated feeding cells, has not yet been found among 

 the adults of the Protozoa ; though, if this is correct, such a dis- 

 covery may be reasonably anticipated. We have proposed to clas- 

 sify these stages under the name of Protembryo. 



(4) The Blastula is in aspect and general characteristics the 

 morphological equivalent of the adults of the genera Volvox and 

 Eudorina, the types of the Mesozoa or Blastrea. The latter are 

 animals in which growth remained permanently arrested at the 

 single-layered, spherical stage in the evolution of tissue-building 

 forms. We have proposed to classify these stages under the name 

 of Mesembryo. 



(5) The Gastrula can be compared, as has been done by Haeckel, 

 with the lower Porifera (Ascones), but these have three laj^ers like 

 the lowest Hydrozoa, in which a three-layered gastrula-like stage 

 has been permanently preserved. 4 The proper name for these 

 stages would therefore be Metembryo, in allusion to the fact that 

 the ovum at this stage is probably essentially a Metazoon. 



(6) The first and simpler planula stages, though often character- 

 istic of the larger divisions of the Animal Kingdom, would not, if 

 arrested m this period, be recognized as belonging to the same 



4 The true two-layered Gastrea type of Flaeckel has, therefore, not been discovered. 

 Doubtless, some such animals bridging the gap in the line of graded modifications be- 

 tween three-layered Ascones and single-layered Volvox will yet make their appear- 

 ance, but we cannot consider any of the animals heretofore described as filling this 

 gap to be entitled to such a position. They have all proved to be either three-layered, 

 or eUe to belong to the true Mesozoa or Protozoa. See also for remarks on the prev- 

 alence of the three layers even in the gastrula, Metschuikoff, " Ueber gastrula einiger 

 Metazoen" Zeitsch. Wiss. Zool., xxxvii, 1882, p. 305. 



