ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 943 



portion of the trunk masses of cells pass down to the yolk by narrow 

 slits, between the gat and the vertical portion of the parietal plates 

 (17th day). There they accumulate beside the embryo, and move 

 peripherally beneath the splanchnopleure. They give rise to the first 

 vessels of the yolk, and especially to the peripheral vein of the area 

 vasculusa. These masses of cells are enveloped by flat cells ; their 

 origin is comparable to a budding of the mother blood-vessel ; from 

 the latter as they grow they derive their contained corpuscles. When 

 the marginal vein has developed and come into connection with the 

 sinus veuosus (19th day), the latter is considerably enlarged, and 

 exhibits a great number of blood-corpuscles. 



Larval Theory of the Origin of Tissue.* — Prof. A. Hyatt en- 

 deavours to prove a phyletic connection between Protozoa and Meta- 

 zoa, and to show that the tissue-cells of the latter are similar to 

 asexual larva?, and are related by their modes of development to the 

 Protozoa. This seems to be indicated by the fact that the tissue-cells 

 exhibit highly concentrated or accelerated modes of development 

 accordiug to a universal law of biogenesis. 



No bushy colonies of cells are built up in the Metazda, except in 

 cases of incomplete segmentation of the ovum. These forms are 

 skipped, and the complex colonies which arise by fission consist of 

 " zoons " divided by distinct walls. As a result of the thickening of 

 the mesembryon, the habit of budding was more or less suppressed, so 

 that the higher types are to be considered as individuals with a 

 highly plastic form, liable to excessive outgrowths, but not as branch- 

 ing Metazoa. 



The author advocates the doctrine of the common but independent 

 origin of types, and urges that paleontology carries back their origin 

 further and further. Early geologic, like early ontogenetic history 

 exhibits a more highly concentrated and accelerated process in evolu- 

 tion than that which has occurred at later periods of the earth's 

 history 



Mechanics of Development^ — Dr. W. Eoux continues the general 

 character of his previous researches in a memoir on the mechanics of 

 development in the embryo, which forms one of the pioneer explora- 

 tions of this but little known field. He distinguishes self-differentia- 

 tion, where the specific nature of the modification is determined by 

 the energies of the system, from correlative differentiation, or change 

 determined by action and reaction between the system and its sur- 

 roundings. Having previously shown, in contrast to Pfluger, that 

 the formal development of the egg is independent of gravity, he notes 

 this as a case where self-differentiation predominates. The influence 

 of electrical stimulus and of mechanical injury at various stages are 

 discussed in detail. Thus frog ova pricked with a needle, so that some 

 of the material was lost, often developed into smaller moribund forms. 

 After considerable loss, however, normal development frequently 



* Proc. Bost. Soc. Nat. Hist., xxiii. (1885) pp. 45-163. 

 t Zeitschr. f. Biol., xxi. (18S6) pp. 1-118. 



