108 



INVEETEBEATA 



CHAP. 



the egg, are far smaller than their sister cells, the so-called fourth 

 quartette of rnicromeres. According to Surface these minute 

 macromeres are destined to undergo ultimate absorption, and take 

 no further part in the development. Their nuclei degenerate, the 

 chromatin in each becoming aggregated into a single deeply-staining 

 mass ; in fact these degenerating nuclei form an excellent land- 

 mark by which the egg can lie orientated. 



Of the so-called micromeres of the fourth quartette, which are 

 really the largest cells in the egg, three, viz. 4a, 4&, and 4c, undergo 



no further development, 

 but form masses of yolk 

 which gradually degenerate 

 and are absorbed by the 

 cells of the gut, when this 

 is formed. The fourth, 4:d, 

 contains the greater part 

 of the original macromere 

 D, and gives rise to the 

 whole of the endoderm 

 and a considerable portion 

 of the so-called mesoderm, 

 that is, the so-called paren- 

 chyma or mass of muscular 

 and connective tissue cells 

 which intervenes between 



FIG. 83. Optical section of developing egg of Plano- 

 cera i/n/nili/to viewed from posterior pole. (After 

 Surface. ) 



7 



mes, the so-called mesodermic bands; N, nuclei of the 

 three inactive cells, 4a, 4/i, and 4r. 



ectoderm and endoderm. 



This cell, 4d, divides 

 into an inner cell termed 

 by Surface 4^ 2 , and an 

 outer, 4:d l . (This nomen- 

 clature is incorrect as 4rf' 2 certainly lies above 4<i 1 .) 4(Z 2 divides 

 into two exactly equal daughters lying right and left of the median 

 plane of the embryo. Just after this the outer cell, Ad 1 , divides in 

 an exactly similar fashion. The inner cells bud off two very small 

 cells with deep-staining nuclei (end', Fig. 84); they lie just above the 

 spot where the pharynx will be formed and are incorporated in the 

 wall of the alimentary canal. 



Their larger sisters divide so as to form each a string of cells 

 directed obliquely forwards on either side (mes, Figs. 84 and 86). 

 These strings or bands are compared by Surface to the similar bands 

 which are found in Aimelidan and Molluscan embryos, which arise 

 from the division of 4rf, and which give rise to the coelorn, but this is 

 doubtful since the cells in question only occasionally take on a band- 

 like form. They give rise, according to Surface, to a large portion 

 of the general parenchyma of the body, but the parenchyma near the 

 lower pole of the embryo is derived from descendants of cells of the 

 second quartette, whicli migrate inwards and supply muscles for the 

 stomodaeum (mes.ect, Figs. 84 and 86) ; this parenchyma is known as 



