618 



INVERTEBRATA 



CHAP. 



blastopore, instead of being directed posteriorly, becomes directed 

 dorsally (Fig. 447) ; in fact, the change which occurs is precisely 



P 



mch 



ms 



FIG. 447. Optical sagittal section of gastrula of Cynthia, partita. (After Conklin.) 



blp, blastopore ; i-li, notoeliord ; inch, mesenchyme ; ms, mesodermal band in vertical position ; 

 n.p, neural plate ; ph, pharynx. 



parallel to the change which 

 takes place during the closure 

 of the blastopore in Amphioxus. 



The column of mesenchyrne 

 cells then becomes broken into 

 two masses, an anterior one adher- 

 ing to the top of the column of 

 7 muscle cells, and a posterior one 

 attached to the foot of this column. 

 Soon afterwards the vertical 

 columns of muscle cells are again 

 forced into a horizontal position, 

 and eventually constitute two rows 

 of muscle cells, one above the 

 other, on each side of the embryo. 

 Each row consists of about six 

 cells, and each cell forms a long, 

 horizontal lozenge-shaped struc- 

 ture, and develops muscular fibrils 

 in its substance. This change, 

 from a vertical to a horizontal 

 position, which occurs in the 

 muscle cells, is due to the growth 

 in length of the hinder end of 

 the embryo, a process which leads 

 to the formation of the tail. The anterior mesenchyme is now situated 

 in front of the rows of muscle cells, and is incorporated in the trunk 



FIG. 448. Dorsal view of an embryo of 

 Cynthia partita after the closure of the 

 blastopore, in order to show the formation 

 of the tail. (After Conklin.) 



mch, mesenchyme which will give rise to the 

 mesoderm of the adult Ascidian ; ms, meso- 

 dermal band drawn out into a horizontal position 

 by the growth of the tail ; n.f, neural folds ; n.p, 

 neural plate. 



