128 COMPARATIVE ANATOMY CHAP. 



The region at the animal pole of the germ (egg) in which the micromeres and some 

 of the merocytes are visible is called the germ or germinal disc. 

 The further course of segmentation is as follows : 



1. The micromeres already formed continue to divide independently. 



2. Towards the edge of the germ disc new micromeres are constricted off from the 

 merocytes, and then associate themselves to those already existing, so that the area 

 of the micromere region, or germ disc, increases. 



3. The merocytes at the edge of the germ disc, as well as those which lie deep in 

 the nutritive yolk, constantly divide, so that not only does the germ disc grow at its 

 edge, but the nutritive yolk becomes more and more crowded with merocytes, which 

 penetrate further and further into it. 



Finally, the germ shows the following structure : at the animal pole, in the middle 

 of the germ disc, lies a flat mass of micromeres consisting of several layers, the rudi- 

 mentary ectoderm. Lying at the edge of the germ disc, and imbedded in the yolk, 

 are merocytes. The greater number of these merocytes yield the material from which, 

 by the formation of micromeres in a way hereafter to be described, a large part of the 

 inesoderm and the endoderm are built .up. By the constriction of micromeres from 

 merocytes at the edge of the germ disc, and by division of the already formed ecto- 

 derm cells, the cap of ectoderm increases still more, so that it grows more and more 

 round the germ as the blastoderm. 



We see that in the development of the meroblastic telolecithal egg the gastrula 

 form is unusually indistinct, because of the enormous mass of the nutritive yolk. 

 The micromere-cap respreseuts the ectoderm ; the yolk with the merocytes the endo- 

 derm and part of the niesoderm. If a blastopore is sought for, this can only be re- 

 presented by that portion of the germ which is not covered by the ectoderm cap, 

 where the nutritive yolk comes to the surface. The edges of the blastopore are 

 represented by the edges of the ectoderm cap. The gastrula formation here also 

 occurs by means of epibole ; the gastrula is a solid discogastrula. 



In the above description only the most important types of segmentation and 

 formation of the two primitive germ layers have been brought forward. The 

 processes in the animal kingdom are in reality extremely varied, yet most of them 

 can be included without much difficulty under one or other of our heads. The great 

 majority of investigations are still insufficient ; because in some cases, as in the 

 meroblastic eggs, observation is very difficult. The difficulty is to establish quite 

 clearly the distribution of the formative yolk in the egg, and to follow the division 

 of the blastomeres in detail. 



The method of gastrulation does not afford any means of recognising the relation- 

 ships of animals, as it is determined by the quantity and distribution of the nutritive 

 yolk, and this may be quite dissimilar in animals nearly related, and similar in 

 widely separated forms. 



The question, which sort of segmentation and gastrulation is the original, has 

 been much discussed. Most authorities hold, not without reason, total equal 

 furrowing, the formation of a cceloblastula, and the subsequent formation of a coelo- 

 gastrula by invagination to be the original method. This view is supported chiefly 

 by the following facts : (1) the want of a nutritive yolk, which cannot have been 

 present in corresponding racial forms ; (2) the similarity of the blastula with certain 

 Protozoan colonies (Magosphcera, Volvox) ; (3) the similarity of the ccelogastrula 

 with the simplest Ccdcnterata (Olynthus, Hijdroids). Just this similarity makes it 

 possible for us the more easily to imagine these germ forms as independent adult 

 animals. 



The free -swimming gastrula, which is usually ciliated on the outer surface, 

 possesses a mouth (blastopore) through which it can or could take food into the 

 enteric cavity. 



