Values of the Stages of Growth and Decline. . 873 
other forms of ova having more or less specialized and concentrated 
_ modes of development may be referred 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 place originated by vertical fission, the Monoplacula ; 
(3) the second stage of segmentation in which two layers 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, primitive endoblast 
cells takes place in the Diploplacula, and the morphological equiva- 
lent 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 discovery may be reason- 
ably anticipated. We have proposed to classify 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 
orms. We have proposed to classify these stages under the name 
of Mesembryo, 
(5) The Gastrula can be compared, as has been done by Heckel, 
with the lower Porifera (Ascones), but these have three layers like 
the lowest Hydrozoa, in which a three-layered gastrula-like stage 
has been permanent] y preserved.! 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- 
to such a position. They have all proved to be either three-layered, or 
else to belong to the true Mesozoa or Protozoa. See also for remarks on 
bi va prevalence of the three layers even in the gastrula, Metschnikoff, 
deoa gastrula einiger Metazoen ” Zeitz, Wissen. Zool., V. 27, 1882, 
