270 



ZOOLOGY 



SECT. 



divides into two cells, an outer (4<f) and an inner (4d 2 ) ; the former 

 divides to give rise to the greater part of the endoderm (end). The 

 latter, dividing into two cells, each of which contributes a small cell 

 (end) to the pharyngeal part of the endoderm, gives rise to two strings 

 of cells (mes) extending obliquely forwards and spreading out to 

 form eventually a continuous layer constituting the greater part of 

 the mesoderm. The mesoderm in the neighbourhood of the stomo- 



dseum is independently 

 developed from descen- 

 dants of the second 

 quartette. 



The process by which 

 the germinal layers 

 have become formed is, 

 as in the Ctenophora 

 (p. 215), a process of 

 epibolic gastrulation. 

 The brain is developed 

 from a pair of thicken- 

 ings of the ectoderm 

 (Fig. 220, g) : these 

 unite into a common 

 mass from which the 

 longitudinal nerves are 

 formed as backward 

 outgrowths. The mouth 

 is developed as an in- 

 growth from the ecto- 

 derm in the position of 

 the former blastopore, 



mac 



FIG. 220. Embryo of Planocera. Diagrammatic 

 frontal section, passing through animal (upper) and 

 vegetative (lower) poles, mes. mesodermal bands 

 (products, with end'., of 4</ 2 ) > ewtf. endoderm 

 (products of MZ); end', small endoderm cells formed 

 from mesoderm rudiment ; wae.-vestigial megameres ; 

 mes. ect. cells of ectodermal origin derived from the 

 second quartette which migrate inwards to form 

 muscles of the stomodanim ; g. cells forming rudi- 

 ment of brain. (From MacBride, after Surface.) 



and, as the embryo 

 becomes flattened, passes from the posterior end to what is destined 

 to be the ventral surface of the worm, while the muscular tissues of 

 the wall of the pharynx are formed from surrounding mesodermal 

 elements. The intestine is at first simple in form ; the cseca are 

 developed as a result of the formation of vertical mesodermal 

 septa which, growing inwards, constrict the enteric wall and the 

 enclosed mass of nutrient material. The embryo, which has 

 assumed an ellipsoidal shape, becomes flattened in the dorso-ventral 

 direction, and, having absorbed the greater part of the nutrient 

 matter, escapes by rupture of the egg-shell. 



In many cases the embryo develops into a characteristic larval 

 form, such as that known as Miiller's larva (Fig. 221). It assumes 

 an oval shape, with a series of eight elongated processes, fringed 

 with long cilia connected together into a continuous ciliated band. 

 There are eye-spots at the anterior end and a mouth in the middle 

 of the ventral surface. The form of the body alters after a time, 



