EAKLY LARVAL DEVELOPMENT. 545 



288, 290, c) does not arise as an epithelial fold, but as a simple 

 thickening due to the ectoderm-cells increasing greatly in height. 



The medullary tube (Fig. 28S. ////■), in the central canal of which 

 backwardly directed cilia can still be seen, has an anterior swelling, 

 in which not only are the walls thicker but the central canal wider. 

 Even in very young stages a pigment-spot is found in the ventral 

 wall of the medullary tube in the fifth metamere (Fig. 288). Later, 

 a similar spot, functioning as an eye-spot, appears at the anterior end 

 of the cerebral swelling (Figs. 289, 290). The posterior end of 

 the medullary tube appears dilated (Fig. 288), and this swelling- 

 contains the undifferentiated material which, as growth advances, 

 continually produces new parts of the medullary tube. This dilated, 

 posterior end is bent round the posterior end of the chorda. At this 

 point the communication with the alimentary canal is found {nen en- 

 teric canal, Fig. 288, en). 



In cross-sections, the medullary tube is found to consist of a 

 single layer of cells (Fig. 234. n), the first perceptible nerve-fibres 

 being found in the ventro-lateral corners, in contact with the chorda. 



The rudiment of the mesoderm consists of the series of consecutive 

 primitivi segments (Fig. 286, us\ us") and of a posterior unsegmented 

 region, the mesoderm-folds (mf) which (according to Hatschek) 

 terminate in the pole-cells of the mesoderm [nip). In this posterior 

 region, the coelom is still for a time in communication with the 

 enteric cavity. Later, however, the mesoderm-folds become com- 

 pletely separated from the entoderm-sac, and, as the posterior end of 

 the chorda-rudiment becomes similarly abstricted from the alimentary 

 canal, the neurenteric canal, in later stages, forms a simple communica- 

 tion between the ventrally-curved posterior end of the medullary tube 

 and the most posterior end of the intestine. 



Even in the stage with eight primitive, segments, we find indica- 

 tions of that asymmetry which affects the later development of the 

 body. The primitive segments of the right side of the body lie 

 somewhat further back than those of the left (ef. the boundaries of 

 segments fully outlined with those in dotted outline in Fig. 287). 

 This asymmetrical shifting of the primitive segments takes place to 

 such an extent that the junction between two segments on one side 

 coincides with the centre of a segment on the opposite side. 



In the stage with nine primitive segments, the most anterior 

 segment sends off dorsally, at the side of the chorda-rudiment, a 

 hollow process (Fig. 286, tn) which gradually grows forward to the 

 anterior extremity of the body. This cephalic process of the mesoderm 



NN 



