86 



REVIEW. 



its disappearance been observed. Heichenbach compares it to 

 the " Dotterkern" of Arachnida. 



Fig. 2. 



Fig. 1. 



Fig. 6. 



Fig. 1.— Part of a section of the Perimorula of Astacus : the protoplasm is 

 distinguished from the deutoplasm by shading, c. Central globular 

 body. y./). Yolk pyramids, e^'. y. White yolk elements. ?;. Nucleus 

 of one of the blastoderm cells. 



I'iG. 2. — Longitudinal section of an embryo in the first stage (PI. VI, fig. 

 1); the endoderm is distinguished from the ectoderm by shading! 

 a. Archenteron. b. Blastopore. ec. Ectoderm. en. Eudoderm. 

 ifiis. Mesoderm. 



Fig. 3.— Longitudinal section of an embryo in the second stage (PI. "VI, 

 fig. 2), showing the forward extension of the archenteron, and the subse- 

 quently iuvaginated plug of endoderm, immediately to the right of b. 



Fig. 4.— Anterior edge of blastopore, in the third stage, showing the origin 

 of the primary mesoderm {p. ms.). 



Fig. 5.— Two of the endodermal cells from the hamal side of the archenteron 

 in the fourth stage, n. Nucleus. _y. Yolk spheres, p. One of the 

 pseudopodial processes. 



Fig. 6.— Three of the endodermal cells from the neural side of the archen- 

 teron in the sixth stage, showing the endogenous origiu of the secondary 

 mesoderm. «. Nuclei, .v. ms. Secondary mesoderm. 

 (Figs. 1—6, after lleichenbach, somewhat diagrammatised.) 



After this preliminary account of the aj)pearauces presented 

 by the egg immediately after the formation of the blastoderm, 

 Reichenbach describes six developmental stages, in the first of 

 which the formation of thcarchenteronhas commenced, while in the 

 sixth the Nau})lius-form is assumed. 



In the first stage (PI. VI, fig. 1), the surface of the blasto- 



