274 C. O. WHITMAN. 



controlled by the pressure of the cells constnntly beiiij; formed 

 by the terminal neuroblasts and mesoblasts. This pressure reacts 

 upon the neuroblasts themselves, especially the two outer ones 

 {x^ a^). That these two cells on each side retreat downward and 

 backward is seen by comparing fig. 40 with fig. 38. The same 

 movement is still more apparent in stage 41, where the two cells 

 {x^) are nearly in contact, the inner cells (.r^ x^) having mean- 

 while moved farther apart in consequence of the invaginatory 

 movement of the blastoineres. At this epoch (figs. 41-42) the 

 anterior ends of the bands are already in contact, and between 

 their concrescent edges is seen the primitive groove (p. gr.) 

 which is continuous with the postoral linear depression. 



In stage 40 the two lines {oc) which show where the bands 

 border upon the cephalic portion, are marked not so much by a 

 depression as by an increasing elevation of the former above the 

 level of the latter. The foremost extremities of the bands being 

 pushed by the cells behind and resisted by the cephalic mass, rise 

 to a height which plainly exceeds that of any other portion. 

 This pressure, furthermore, causes them to expand a little on 

 each side of the head-portion. In stage 41, in consequence of 

 the concrescence of the fore ends of the bands, the lines of abut- 

 ment {oc), instead of lying nearly parallel with the postoral 

 depression as in fig. 40, lie almost at right angles to the same and 

 now form well marked linear depressions, the distal ends of which 

 are rounded. The raised extremities of the bands form the 

 first somatomere of the embryo. At this time the expanded 

 blastodisc, the marginal bands of which alone are seen in my 

 figures, covers about one half of the q^^. 



As the bands lengthen the concrescence along the median 

 ventral line continues until finally they are united from end to end 

 (fig. 48). The somatomeric division of the embryo follows 

 closely upon the closing of the blastopore, progressing from the 

 cephalic towards the anal end. 



The embryo leaves its protective envelope soon after stage 47 

 (72 h.) and becomes attached to the ventral side of its parent, 

 under whose protection it remains until it is fully developed and 

 able to seek its own food. The point of attachment is a place on 

 the neural side of the embroyo, just behind that part destined to 

 form the anterior sucker. ^ 



In what manner the attachment is ell'ected I am unable to 

 say. Eiiil)ryos taken from the j)arent at the time of exclusion 

 almost invariably unite in pairs, and the place of contact is 

 always that by which they are attached to the parent. lu such 

 cases they adhere so strongly, that they are generally injured by 



' Ilofl'iiiann (-^) tliinks the embrjo attaches itscH" by the I'ulurc suc- 

 torial builacc ilbcir. 



