No. r.] GERM-LAYERS IN" CLEPS/ISTE. 113 



The right mesoblast x thus becomes imbedded mainly in 

 c, but usually lies partly in b. Meanwhile, the mesoblast, xy, 

 takes up an opposite position in the left macromere, a. I have 

 never seen a single case in which bilateral symmetry was com- 

 plete with respect to the mesoblasts ; but the final position is 

 always such that each proliferates exclusively for the germ- 

 band of its own side. Their relative positions in transverse 

 section are shown in Fig. 2, PI. IV. In Fig. 3, representing 

 a sagittal section of C. complaiiata, the right mesoblast, x, is 

 placed much farther forward than is usually the case in C. 

 marginata. 



By successive vertical cleavages the neuro-nephroblast is 

 split up into eight octoblasts, arranged symmetrically in two 

 groups at the posterior edge of the blastodisc, as shown in 

 Diag. 4. These cells have a superficial position at first, but 

 the two median ones are very soon covered by the expanding 

 blastodisc, and the rest are later overgrown by the same 

 elements. 



Towards the close of this period, when all arrangements for 

 the formation of the embryo have been completed, free nuclei 

 begin to appear in the surface of the three entoblasts, a, b, c. 

 The origin and fate of these " entoblasts " have been traced 

 with considerable care, and the results of my study on this 

 point have confirmed the opinion that they give rise to the 

 mesenteron. 



II. ORIGIN OF THE MESENTERON. 



I. Historical and Comparative. 



Clepsine. — Grube (No. 2), and Leuckart and Rathke (No. 3), 

 derived the entoderm by delamination from the blastoderm. 

 But these authors, whose observations on this subject were made 

 long before the introduction of the microtome, were not able to 

 bring any direct evidence of such a mode of origin. It was 

 simply the most rational conclusion open to them, considering 

 the methods at their command, and what was then known 



(2.) Grube, A. E. Untersuchungen iiber die Entwicklung der Clepsinen. K'onigs- 

 berg. 1884. 



(3.) Rathke, H., and Leuckart, R. Beitrage zur Entwicklungsgeschichte der 

 Hirudineen. Leipzig. 1 862. 



