THE DEVELOPMENT OF THE EARTH-WOKM. 239 



the region where the cords are already distinctly separated, 

 some adjacent ectoderm cells may assume their specific 

 characters and join them. 



From this time the cells of the neighbouring borders of the 

 cords force themselves under the furrow, slightly raising 

 the ciliated cells. They approach the median line, and there 

 those of the two sides unite ; thus, the two primitive lateral 

 cords join to form a single lamina, which I shall call the 

 ventral medullary plate, and soon afterwards its cells begin 

 to accumulate at certain points, producing a successive 

 series of zones, alternately alike and unlike. This, as 

 well as what follows, will be best explained by reference 

 to successive transverse sections. Only perfectly vertical 

 sections are of use ; in these the sections of the longitu- 

 dinal muscular fibres appear as circular points 



In the section fig. 25 a, which, together with 25 b and 25 c, 

 is taken from the tail end of an embryo 3'0 mm. in length, 

 the junction of the cords has taken place. Beneath the 

 bottom of the furrow S7i the medullary plate {n) consists of 

 a single layer of cells, but is raised immediately to the right 

 and left into two parallel crests, which, becoming gradually 

 lower towards the sides, terminate in a thin lamina. The 

 dorsal surface of the plate is nearly flat, and is covered by a 

 thin layer derived from the somatic lamina [ho). Thicken- 

 ings of the somatic lamina are seen on each side of the plate, 

 and between them and the ectoderm the rudiments of the 

 muscular plates {m) ; above the medullary plate, projecting 

 into the body cavity, is the ventral blood-vessel (tj), attached 

 to the splanchnic lamina (from which it takes its origin), 

 that envelopes the mid gut (en). The section 25 b, which 

 immediately folloAVS the last, shows a different arrangement; 

 here the conspicuous elevations at the sides of the furrow are 

 wanting, and the medullary plate is reduced nearly every- 

 where to two layers of cells ; but in the third section (fig. 

 25 e) it has again the form and extension which it had in 

 25 a. Such a succession of thick and thin zones is repeated 

 many times, with the difference, however, that further for- 

 ward the size of the thick zones is greater, so that they 

 occupy two or three sections instead of a single one, and the 

 diff'erences between the zones become less marked. 



On examining a series of sections taken from the middle 

 of the body of the same embryo, the first thing which strikes 

 one is the great enlargement of the medullary plate, which 

 in this stage has, in fact, attained its largest relative dimen- 

 sions. The ventral furrow has disappeared, and its cells, 

 although still recognisable, have greatly changed their 



