NOTES ON EMBRYOLOGY AND CLASSIFICATION. 



403 



ceivable mechanical causes, than it is to marshal the same 

 series of facts in accordance with Haeckel's doctrine of the 

 invaginate mouth-bearing Gastrsea. 



The histoi'ic series according to the Planula theory. — 

 Accordingly, I shall briefly pass in review what I conceive 

 to have been the course of historical development, pointing 

 out how these historical phases reappear iu more or less 

 modified forms in the embryonic histories of to-day. 



1. The monoplast = ovum. — The unicellular ancestors of 

 the higher animals are represented by the unicellular ovum. 

 Just as we find existent unicellular animals exhibiting 

 diff'erentiation into ectoplasm and endoplasm, so do we find 

 ectoplasm and endoplasm differentiated in many eggs. The 

 differentiation of anterior and jDOsterior regions, which is 

 more rarely seen in living Protozoa, is the rule in the mono- 



Ficj.J^ 



Fig. 1. Monoplast. Fig. 2. Optical section of Morula stage. Fig. 3. 

 Optical section of Diblastula. Fig. 5. vSection of Diblastula, with mouth. 

 Ec, Ectoderm. En, Endoderm. M, Month. F, Food particles. 



plastic phase of individual development. Just as unicellular 

 animals contain granular matter, which as metaplasm is dis- 

 tinguished from the hyaline protoplasm iu which such 

 granules float, so in the egg-cell we find a greater or less 

 amount of granular matter, which it is convenient to speak 

 of as food-material. As in the unicellular organism some of 

 the granular matter present, is the result of chemical activity 

 in the protoplasm — that is to say, is the product of assimila- 

 tion and subsequent segregation, whilst other (usually coarser) 

 particles are particles of food which have been incepted, but 



VOL, XYII. NEW SER. D D 



