CLEAVAGE AND FORMATION OF THE GERM-LAYER. 



3 



4,OPo2 



A, e). The caudal prominence described by Metschnikoff (No. 24) is 

 probably to be identified with this growing point ; the former projects into the 

 yolk, and at a later period shifts into the caudal region of the embryo. Laukie 

 compares the thickened part with the primitive streak in vertebrates, and we are 

 again involuntarily reminded of the conditions found in Peripatw. In the latter 

 the "point of ingrowth" lies at the posterior end of the long blastopore.* 



When the germ-disc, by the active increase of its elements, lias 

 attained a thickness of several cells, these still appear but slightly 

 differentiated into 



layers. The inner rf( 



surface of the germ- 

 disc is now quite 

 irregular, for single 

 ■cells become de- 

 tached from it, and 

 shift into the yolk 

 <Fig. 2, B). These 

 cells give rise to 

 the amoeboid yolk- 

 cells, which are dis- 

 tributed throughout 

 the yolk, and bring 

 about its disinteg- 

 ration, without, how- 

 ever, taking part in 

 the formation of the 



embryo (Kowalevsky and Schulgin, Laurie, [Brauer]). They thus 

 differ from the corresponding cells in the Araneae, which participate 

 in the formation of the enteron. The entoderm of the Scorpiones 

 arises by the differentiation of the cells of the germ-disc lying next 

 to the yolk to form a regular epithelium (Fig. 3, A and B, ent.). 

 The cells of this layer differ further from the adjacent cells by their 

 highly-refractive appearance, due to the fluid yolk which they have 

 absorbed. 



The mass of cells which, after the differentiation of the entoderm, 

 remains between it and the ectoderm, corresponds to the mesoderm 



* [Brauer (App. to Lit. on Scorpiones, No. II.), in his interpretation of this 

 posterior thickening of the blastoderm, disagrees with all former investigators ; 

 he sees in this thickening the genital rudiment. The mesoderm-cells arise in 

 front and at the sides of this thickened area as proliferations from the ectoderm. 

 The entoderm arises by delamination from the primary blastoderm, and is first 

 observed between the yolk-cells on the one hand, and the ectoderm or the genital 

 rudiment on the other. — En.] 



Fig. 2.— A and B. Sections through the germ-disc and part of 

 the yolk of Euscorinvs italicus (after Laukie). d, yolk ; fes, 

 germ-disc. 



