THE DEVELOPMENT OF THE EARTH-WORM. 231 



layers keep their primitive state nearly unaltered. The en- 

 doderm shows no other change than the enlargement of its 

 cells filled with numerous granules of dense albumen, and 

 the displacement of their small oval nuclei towards the free 

 surface. The reproductive activity of the ectoderm appears 

 to be confined to the production of the secondary epithelium 

 of the head intestine -, in its other parts the cells become 

 very much more stretched out into thin plates by the in- 

 creasing internal pressure, which is greatest on the dorsal 

 surface, where they become so thin that it is sometimes 

 difficult to recognise them. They retain, however, their 

 nuclei, placed in small thickenings, which project inwards, 

 taking advantage of the less resistance at the lines of separa- 

 tion of the endoderm cells. 



But when the anterior zoonites are marked out, the 

 ectoderm resumes its reproductive activity, the first and 

 most important result being the formation of the central 

 nervous apparatus. 



Development of the Cephalic Ganglion. 



The investigation of the first stages of the development of the 

 supra-oesophageal or cephalic ganglion is rendered specially 

 difficult by the rudiment being situated on a strongly 

 curved projection. In investigating the difierentations in a 

 very small space, and of a tissue composed of very small 

 cells, only the very thinnest possible sections are of use, 

 which, to render the relations of the surrounding parts in- 

 telligible, must pass exactly at a right angle through one of 

 the principal axes of the rudiment, a condition which can 

 only be obtained by chance in transverse sections ; in 

 longitudinal sections the median one is vertical, but all 

 the others are necessarily oblique ; this is even more the case 

 with horizontal sections. But since there is no other method 

 of research, I have made sections in all directions ; by 

 combining the sections of a series with one another, and 

 with those of other series made in different directions, I 

 think I have formed a fairly precise conception of the way in 

 which the cephalic ganglion is formed. 



Fig. 23, Pi. X, represents the anterior part of the exactly 

 median section of a longitudinal series, made from an embryo 

 of about 0*4 mm. in length. The structure of the cephalic 

 ring, already described, is easily recognised ; the head cavity, 

 lined by the large splanchnic lamina ilsp) and by the so- 

 matic lamina {Iso'), here reduced to a very thin layer of fusi- 

 form cells. The ciliated epithelium of the mouth (eo) is 

 folded towards the external dorsal surface, where it becomes 



