PLATYHELMINTHES 



109 



mes 



mesectoderm in order to distinguish it from the parenchyma derived 

 from 4d 2 which occupies the more dorsal portion of the embryo, and 

 which is termed mesoderm. 



The endoderm, with the exception of the two small cells budded 

 from 4:d 2 , is all derived from the divisions of 4^. This cell, as we 

 x have seen, divides into 

 right and left sisters ; 

 each then divides 

 several times so that 

 six or eight large cells 

 are formed, lying at the 

 posterior pole of the egg 

 (end, Fig. 84). Soon 

 afterwards the spreading 

 edge of the ectoderm, 

 consisting of the 

 daughters of the second 

 and third quartettes of 

 micromeres, reaches the 

 lower pole of the egg 

 and covers this- group 

 of endoderm cells. 



During this time the 

 other three micromeres 

 of the fourth quartette 

 have remained un- 

 divided, but by the 

 pressure of the daughters 

 of Aid 1 they are pressed 

 higher up in the egg ; 

 their yolk-granules begin to coalesce so as to form fat-like drops, 

 and these drops run together so as to form a limited number of 

 enormous spherules. The nuclei remain large and conspicuous 

 (N, Fig. 83). 



When the ectoderm has completely covered the egg, an invagi- 

 nation takes place at the lower pole; this is destined to form the stomo- 

 daeum of the larva. The four minute "macromeres" become pushed 

 in before it and ultimately disappear altogether. Then the mass of 

 cells derived from ^d 1 begins to separate one cell from another, in the 

 middle, so as to give rise to a cavity, the future gut-cavity. 



As the endodermic cells multiply they spread in an amoeboid 

 fashion over the outer surfaces of the yolk-spheres derived from 4, 

 4&, and 4c, so that these are included within the lumen of the 

 alimentary canal and are absorbed (Fig. 86). But in Discocoelis, 

 according to Lang, 4 and 4& and 4c bud off small cells which take 

 part in the formation of the alimentary canal, so that in Planocera 

 one member of the quartette does the work which in Discocoelis is 

 done by all four members. This kind of variation is not uncommon 



mac 



FIG. 84. Diagrammatic frontal section through egg of 

 Planocera inquilina at a later stage of development 

 than that represented in Fig. 83. (After Surface.) 



rues, mesodermic bands ; end, endoderm ; end', small endo- 

 dermal cells formed from mesoderm rudiment ; mac, vestigial 

 macromeres ; mes.ect, cells of ectodermal origin which migrate 

 inwards to form muscles of the stomodaeum or larval pharynx ; 

 g, cells forming rudiment of brain. 



