316 SUMMAKY OF CURRENT RESEARCHES RELATING TO 



" The differentiation of the later plates of mesoblast of the trunk 

 directly from the hypoblast is again a fundamental feature of verte- 

 brate embryology, occurring in all types from AmpMoxxis upwards, 

 the meaning of which has been fully dealt with in the ' Treatise on 

 Comparative Embryology ' by one of us. Lastly, the formation of 

 the notochord from the hypoblast is the typical vertebrate mode of 

 formation of this organ, while the fusion of the layers at the front 

 end of the primitive streak is the universal fusion of the layers at 

 the dorsal lip of the blastoj)ore, which is so well known in the lower 

 vertebrate tj^pes." 



Development of Lepidosteus.* — Prof. F. M. Balfour and Mr. W. N. 

 Parker state that the ovum is invested by a thick inner membrane, 

 and an outer layer of pyriform bodies, which would seem to be metamor- 

 phosed follicular epithelial cells ; the segmentation is complete, though 

 very unequal ; here, as in the division of the epiblast into an epidermic 

 and a nervous stratum, and in the formation of the walls of the 

 brain, &c., from a solid " medullary keel," we have resemblance to the 

 Teleostei ; the same is true of the archinephric duct, which is developed 

 from a hollow ridge of the somatic mesoblast, and, by constriction, 

 gives rise to a duct with an anterior pore, leading into the body-cavity. 

 The olfactory sacs arise as invaginations of the nervous layer of the 

 ejjiblast, the superficial epidermic layer becoming ruptured to allow 

 of communication with the exterior ; the primitive single opening 

 divides to give rise to the double opening of the adult. The suctorial 

 disk of the larva is shown to be formed of papillae composed of 

 elongated epidermic cells, which probably pour out a viscid secretion. 

 The pronejjhric chambers remain in communication with the body- 

 cavity by two richly ciliated canals ; some of the mesonephric tubes 

 of the larva have peritoneal funnels. No traces of a hyoid gill were 

 detected in any larvas. 



Spermatog-enesis in Vertebrates and Annelids.j — A. Sabatier 

 considers that the observations he has made on spermatogenesis in 

 Salmacina, one of the SerpulidaB, throw great light on the process in 

 Vertebrates. 



The spermatospores, or mother-cells, which line the walls of the 

 spermatic sacs, are, by multiplication of tlie nuclei and by budding, 

 covered with claviform pedunculated cells, the protospermoblasts. 

 Each of tln'se enlarge, detacli themselves from the group, and in their 

 turn present a new multiplication of nuclei with superficial budding. 

 Hence arises a socond generation of spermatoblasts, the deutospenno- 

 hlasts, which are ultimately transformed into spermatozo'ids, the nuclei 

 of the former forming the heads of the lattei', while the body and tail 

 are filaments of the protoplasm. 



This double generation appears to the author to explain, simply 

 and rationally, the complicated and very extraordinary process attri- 

 buted by Balbiaui to the process of spermatogenesis in vertebrates. 

 The cellular groups composed of a large round central cell (female 



* Proc. Roy. Soc, xxxiii. (1881) pp. 112-9. 



t Compter Roiuhis, .xciv. (1882) pp. 172-3. 



