II 



SEGMENTATION AND GASTRULATION 



125 



yield a great part of the 'mesoderm (musoixlature, sexual organs, connective tissue), 

 and are overgrown by the ectoderm micromeres. All micromeres which are detached 

 later belong to the endoderm. We have here before us a typical case of the tendency 

 to shift back formations to very early stages of development. A typical gastrula is 

 not developed in the Polyclada, since the 

 separation of ectoderm and endoderm occurs 

 as early as the 8-blastomere stage. We 

 shall return to this later. 



The Ctenophora exhibit an interesting 

 process of segmentation and gastrulation 

 intermediate between the gastrulation by 

 means of epibole of the teloleoithal holo- 

 blastic eggs and the gastrulation by de- 

 lamination of the oentrolecithal eggs to be 

 spoken of later. The 8-blastomere stage of ^^S (°f DlsoooeUs). „u. First generation of 

 f . T T , 1 -1 1 T micromeres (ectoderm - forming cells) ; mii, 



these animals has been described above, geeond generation of micromeres (mesoderm 

 All the 8 blastomeres are teloleoithal, with micromeres) ; ma, maoromeres. 

 the formative yolk directed towards the 



animal pole. What occurs at the 4-blastomere stage of the teloleoithal holoblastic 

 eggs hitherto described, viz. the constriction of the 4 micromeres from the 4 maoro- 

 meres, here takes place one stage later, at the 8-blastomere stage. The 8 blasto- 

 meres in fact give off 8 micromeres towards the aboral pole (Fig. 95, A). The further 

 segmentation is quite similar to that of Bonellia. The micromeres increase in 

 number (1) by division, (2) by the continual addition of new micromeres towards 

 the vegetative pole, by constriction from the maoromeres {B, 0). After the micro- 

 meres have thus grown round the maoromeres, leaving a large region at the vegeta- 

 tive pole in which the maoromeres come freely to the surface, the formation of 

 micromeres does not cease, as in the Polyclada. The already formed micromeres, 

 however, yield exclusively ectoderm ; the remaining maoromeres, part of the mesoderm 

 and the endoderm. Here also no very recognisable gastrula is developed. 



Fio. 94.— Segmentation of a. Polyclad 



Pig. 95.— Three stages in the segmentation of a Ctenophoran egg. mi, Micromeres ; 

 ma, macromeres. 



In all processes of the formation of micromeres the following is to be specially 

 noted. After a maoromere has constricted off a mieromere, or, what is the same 

 thing, after a blastomere has divided into a small mieromere with little or no nutri- 

 tive yolk, and into a large maoromere with mueh nutritive yolk, the portion of 

 formative yolk or protoplasm which remains in the macromere grows evidently by 

 the assimilation of nutritive yolk before the macromere can again divide. 



3. Segmentation and gastrulation of the holoblastic oentrolecithal germ» We 

 give as an example of this the Geryonid gei-m which has been the most carefully 

 investigated and is the best understood (Fig. 96). We are already aequainted with 

 the 8-blastomere stage. Each blastomere is teloleoithal, with deutoplasm directed 

 towards the centre of the germ and protoplasm towards the circumference. The 

 8 blastomeres divide into 16, and then into 32 blastomeres of equal size, which 



