874 SUMMARY OF CUERENT RESEARCHES RELATING TO 



Before the completion of tlie gastrula-stage certain changes have 

 taken place in the region of the blastopore, which are connected with 

 the formation of the mesoblast ; the blastopore loses its rounded form 

 and becomes a narrow and deep cleft, which lies in the long axis of 

 the embryo. On the surface of the egg there appear three grooves ; 

 on either side two semilunar grooves gradually increase till they 

 unite ventrally with one another. In front of the blastopore the 

 surface of the egg is depressed to form a small groove which lies in 

 the long axis of the egg, and corresponds to the primitive groove of 

 the Amniotic Vertebrates. A study of the sections made during this 

 period has convinced the author that the mesoblast-stripes always 

 arise in the form of at least two-cell layers, one of which passes into 

 the chorda-endoblast and the other into the enteric-endoblast. No 

 elements are given off from the endoblast, for this, even in the gastrula- 

 stage, forms a single layer of cylindrical cells. The mesoblast does 

 not arise in the form of isolated cell-aggregates, but as two masses 

 of connected cells; these bands first appear in the region of the 

 blastopore and on either side of the chorda-endoblast, whence they 

 gradually extend over the surface of the egg, and, uniting, separate 

 the two primary germinal layers. This layer owes its origin to a 

 paired outgrowth of the endoblast, which commences to be formed 

 before the gastrula-stage is completed ; and, in short, we see that in 

 the triton, as in the other groups previously cited, the archenteron is 

 by two folds broken up into a central space, which forms the per- 

 nianent enteron, and into two lateral diverticula or body-sacs. 



The third period is characterized by the formation of the chorda 

 dorsalis and the separation of the two mesoblast-sacs from the primary 

 endoblast. 



The fourth period of the development of the mesoblast is charac- 

 terized by the formation and growth of the primitive vertebrae and 

 the differentiation of the musculature of the body. The former 

 shows that there is a very intimate connection between the closure 

 of the enteron on its dorsal surface, the separation of the two mesoblast- 

 sacs from the endoblast, and the development of the chorda dorsalis 

 from the chorda-endoblast. In the latter the two coelom-sacs, by a 

 process of successive foldings, give rise to the primitive vertebrae ; this 

 commences in the cervical and gradually extends to the caudal region. 

 The epithelial wall of the coelom gives rise to transverse folds, and the 

 successive diverticula thus formed communicate for some time with 

 the sac of the ccelom ; later on, these diverticula become completely 

 shut off, and then form saccules on either side of the notochord ; in 

 other words, these primitive cavities are parts cut off from the primary 

 coelom. 



Experiments on the Origin of the Difference between the 

 Sexes.* — Dr. G. Bern's experiments were made on the brown grass- 

 frog, ova being artificially fertilized and placed in twenty-one distinct 



* BreBlauer arztliche Zeitscbr., 1881, No. 3 & foil. Cf. Kosmos, viii. (1881) 

 pp. 65-6. 



