SUMMAKY OF CURRENT RESEARCHES. 151 



Ijmph is poured, and it consequently becomes the seat of assimilation 

 and growth. 



The true jolk-nucleus is derived from the centrosome of the dividing 

 oogonia. It does not arise from extruded chromatin, nor from migrat- 

 ing nucleoli, nor from leucocytes or devoured cells. It is the morpho- 

 logical and physiological centre of the cytoplasm. Its strange forms are 

 due to the increasing amount of yolk-granules in its neighbourhood, to 

 the formation of vacuoles and the resulting compression of the concen- 

 tric zones, and to the variable tension of the astral rays. 



Early Developmental Stages in Vertebrates.*— Alexander Meek 

 argues that in Chordates there is a primitive gastrula from which the 

 anterior portion of the head, and of the brain, and a certain part of the 

 ventral region are derived ; that the segmental structures posterior to 

 this region are added by the dorsal lip or the area of the blastopore 

 called by that name ; and that an unsegmented ventral part is yielded 

 by the ventral area of the blastopore, which also affords the mesenchyme 

 of the vascular area. 



He suggests that at an early phylogenetic period the increase in size, 

 which accompanied vertebrate evolution, was met by an accumulation of 

 yolk in the endoderm, and the effect of this was to prevent invagination. 

 The further demands upon the eo;g to provide for a still larger embryo 

 led to the meroblastic conditions illustrated by the Selachii and the 

 Teleostei, in which the yolk part of the egg is altogether unsegmented, 

 and in consequence the " yolk plug " widely distends the blastopore. 

 This brought about the rearrangement of the blastopore, the ventral 

 contribution to the embryo and to the vascular mesenchyme having to 

 be made nearer to the dorsal lip. The manifestation of this is still 

 more plainly visible in Sauropsida and Mammals. The eggs of the 

 latter were without doubt derived from meroblastic forms. 



Embryonic Hydrocoel in Fishes.f — L. v. Betegh describes a 

 disease of trout-embryos, in which the yolk-sac is enormously enlarged 

 and filled with a serous fluid. It appears to be due to a microbe, 

 which is present in enormous numbers in the embryos. For this the 

 name Diplohacillus liquefaciens piscium is proposed. Another disease, 

 tympanitis embryonalis, is seen in young trout of 10-12 days. It is 

 marked by an extraordinary swelling of the abdominal cavity, which 

 becomes filled with gas. The body becomes curved, the fishes swim 

 upside down, the abdominal wall bursts. The disease is probably due 

 to a gas-producing microbe. Double infection, with hydrocoel and 

 tympanitis, may occur. 



Development of Ear in Anamnia.J — Jaromir Wenig has studied 

 this in dogfish, trout, toad, and Pelo'haUs fusca. In Selachians the 

 so-called ductus endolymphaticus is the duct formed by the insinking 

 of the ear, and it remains as the connexion between the ear and the 



* Proc. Durham. Phil. Soc, iv. (1912) pp. 129-33 (1 pi.). 



t Centralbl. Bakt. Parasitenk., Ixvi. (1912) pp. 284-6 (1 pi. and 1 fig.). 



X Morpbol. Jahrb., xlv. (1913) pp. 295-333 (3 pis. and 8 figs.). 



