THE MESODERMAL STRUCTURES. 199 



English authors, in accordance with the terminology used in the Vertebrata, 

 have called the growing zone the primitive streak, and the groove-like depression 

 that occurs in it the primitive groove. If such a groove occurs, it must no doubt 

 be regarded as a continuation of the blastopore, and we must assume that it is not 

 the most posterior part which is retained as the anus. The growing point itself 

 must be considered as lying on the posterior margin of the blastopore. At this 

 point, a great accumulation of cells takes place, and the germ-layers are here still 

 fused and undifferentiated. In so far as the mesoderm-bands extend forward 

 from this undifferentiated cell -mass, the condition here to a certain degree 

 resembles that in the Annelida. Sedgwick even speaks of polar cells of the 

 mesoderm, but of these nothing definite is known. There can be no doubt that 

 the mesoderm is chiefly produced from behind, i.e., from the growing zone, but 

 in consequence of the close apposition of the mesoderm-bands to the edges of 

 the blastopore, the participation of the latter in their growth cannot be excluded 

 (Sedgwick). In the American species, it appears certain that no such partici- 

 pation occurs. The forward growth of the mesoderm-bands takes place from 

 the point of ingrowth, which must be regarded as the blastopore, and their 

 growth determines the lengthening of the whole embryo. The mesoderm-mass 

 here separates from the sac-like rudiment of the enteron (Figs. 100 and 101), 

 but not so sharply as to exclude a connection of the former with the ectoderm 

 on the one side and with the entoderm on the other, which can be proved to 

 exist even at later stages, when the mesodei'm has become far more highly 

 differentiated. The mesoderm may thus be regarded even in this case as 

 arising on the boundary between the ectoderm and the entoderm. 



The further development of the mesoderm-bands takes place in a 

 manner very similar to that in which they develop in the Annelida. 

 Before they have reached the anterior end of the blastopore, they 

 break up into paired, regularly arranged segments (Fig. 99 A-C, 

 us). Cavities then appear in these, and, as these gradually widen, 

 the cell-material of the separate segments becomes arranged into a 

 regular epithelium. The paired primitive segments thus arise. As 

 they extend further, the outer wall becomes applied to the ectoderm 

 and the inner to the entoderm (Fig. 100), like the somatic and 

 splanchnic layers of the Annelida. A pair of primitive segments 

 belongs to each body-segment. The differentiation of the primitive 

 segments commences in the most anterior part of the mesoderm-bands 

 and extends backward, their number increasing with the growth of 

 the body ; the first pair of primitive segments to develop is thus that 

 belonging to the cephalic segment, and this is also much larger than 

 any other pair. It extends almost to the ventral and dorsal middle 

 lines ; the two halves, however, do not come into contact, and con- 

 sequently no mesentery is formed. Transverse sections through the 

 body of an embryo at the stage when the primitive segments are 

 being differentiated closely resemble, especially in the anterior and 

 posterior regions, similar sections through an Annelidan embryo ; 



