256 



PHYLUM ANNELIDA 



wards, but divergent, because of the flattening of the blastula. The 

 endodenn now becomes concave, and thus the blastopore arises, 

 occupying the whole of the lower surface (Fig. 137 {3)). The sides close 

 in and the blastopore becomes a slit, which further closes from behind 

 forwards, leaving only a small opening — the future mouth. During 

 these processes the cells at the anterior tip of the blastopore, which 

 will give rise to the pre-oral lobe, undergo no change, but the mesoderm 

 has been active. 



As gastrulation proceeds, the mesoblast rows grow forwards and 

 upwards, until they come near each other above the anterior tip of the 

 blastopore, while their middle portions are carried downwards until the}' 

 lie on the ventral surface. Over them the ectoderm is thickened in two 

 bands. Two longitudinal rows of ectoderm cells near the anterior end, 

 and ending behind in large cells, sink in just as the primary mesoblasts 

 did. The thickening now extends ventrally until the two bands meet 

 and, passing into the blastopore, form the stomoda^um. Even before 

 this the embryo has begun to swallow the albumin in which it floats. 



There are now two lateral bands of cells called the germ-bands, 

 composed of three layers (Fig. 137 (4)) ; outside is the thickened ecto- 

 derm, next the rows of cells which sank in, and innermost the meso- 

 blast rows. The mesoblast rows have met in the middle line by 

 dividing and widening out into a pair of flattened plates, but they still 

 end behind in the two primary mesoblasts. Coelomic cavities develop 

 in the plates, and the anterior ends meet above the mouth. The 

 ectodermic rows which sank in (there were eight of them, four on each 

 side of the median line, and each ending in a large mother-cell) go on 

 growing. The mother-cells are apparently carried backwards as the 

 embryo lengthens, leaving a trail of daughter-cells behind them. The 

 cells so formed also divide, the embryo rapidly lengthening and finally 

 becoming vermiform. Of the eight rows the innermost on each side 

 (neuroblasts) give rise to the nervous system, the next two rows on each 

 side (nephridioblasts) form parts of the nephridia (Fig. 137 (4)), while 

 of the fourth row nothing definite is known. Each row, ending behind in 

 a single cell, widens out and deepens as it is traced forwards. The neural 

 and mesoblastic rows can be traced round the mouth, and help to form 

 the prostomium ; the others fade away at the sides of the stomodaeum. 



Let us sum up this complex history : — 



''(a) The original outer layer 



becomes the epidermis. 

 (b) The secondary inner strat- 

 um consists of neuroblasts 

 Ectoderm. • which form the nervous 

 system, of nephridioblasts 

 which form parts of the 

 nephridia, and of lateral 

 cells of unknown function. 



Fertilised 

 oviun. 



Blastula. 



Two-layered 



gastrula 



with primitive 



mesoderm 



cells. 



Mesoderm 

 fonned from 



the division of 

 the primitive 



" mesoblasts." 



/"Muscle. 



I Blood vessels. 



"j Inner parts of nephridia. 



' Reproductive organs. 



Endoderm. 



/ Lining of 

 \ mid-gut. 



