272 e. O. WHITMAN. 



rings is the same as that of the white borders of the rings and 

 ring- discs. 



I have seen these nuclei pass through the successive forms of a 

 dividing amphiaster. They multiply rapidly, and in stage 38 

 (30h.) are scattered over the whole outer surface of the blasto- 

 meres. In stage 40, and following stages, they can also be seen 

 on the upper faces of a, c, and b, through the thin ectodermal 

 layer. By the time the germ-bands are fully united, they are 

 very numerous, and much smaller than at first. 



Such nuclei have been observed in the egg of Nephelis 

 according to Balfour (jVy). "Dr. Kleinenberg has followed a 

 single egg through the whole course of its development, and 

 concludes that the nuclei of Nephelis never become the nuclei of 

 new cells. ^^ With reference to Clepsine, I have come to a very 

 different conclusion, as will appear when I come to speak of the 

 origin of the entoderm. 



Whence come these nuclei ? In stage 35 they are not to be 

 seen. A horizontal section of this stage (fig. 80) shows that 

 each blastomere possesses a single nucleus. The nucleoplasm 

 lias a somewhat stellate form ; the rays vary in length, some- 

 times reaching to the irregular circular outline of the nucleus. 

 The same condition has been described for Nephelis by Biitschli 

 (30, fig. 5, PI. XVIII.). rig 61 represents one of these nuclei in 

 a little earlier phase. The nuclei now lie nearer the inner than the 

 outer faces. Fig. 83 represents a horizontal section of the stage 

 37 (nearly), which passes beneath the neuroblasts and the blas- 

 todisc. Here only two nuclei were hit, but these lie near the 

 outer faces of the blastomeres. The nuclei of the blastomeres 

 then pass from their original central position to the periphery, 

 and can here be seen on the living egg. They are much more 

 distinct in C. complanata than in d marginata. 



V. Gastuula and Neitrula. 



The Neurula (Rauber) arises by the concrescence of the thick- 

 ened rim of the blastopore (Lankcstcr). It follows the (jlastrula, 

 j but takes its origin with the germ-bands, i.e., long before the 

 ' Gastrula-phases (invagination, &c.,) are completed. This is 

 a good illustration of ontogenetic concentration — the earlier 

 phases of one stage appearing before the later phases of the 

 previous stage are completed. 



(«) Growth of the Germ-bands and concomitant Invagination. 



— In stage 3S the blastodisc has a quadrilateral form with 



rounded corners, and is bounded on either side with thickened 



margins, the germ-bands (g.b.). Each band results, as before 



, indicated, from the confluence of five streams of cells, and these 



I streams — if the coiiii)arison be allowed — all flow I'lum behind 



