FORMATION OF THE GERMINAL LAYERS IN TELEOSTEI. 227 



cells which are ready for tissue formation arrange themselves into the three 

 primary layers, as in meroblastic ova, and constitute the archiblast. The cells 

 not yet ready, and overladen with nutritive yolk, bud off later processes of 

 protoplasm containing nuclei, which give rise to the parablastic elements of the 

 embryo. 



Waldeyer admits that parablast cells may take part in the formation of 

 the hypoblast in some forms, as has been maintained by so many authors ; 

 but thinks its chief function in the higher vertebrates, at any rate, is 

 to elaborate those cells which give rise to the blood and connective tissue 

 elements. 



Kollmann (17) maintains that the layer which gives rise to the blood and 

 connective tissues represents a distinct advance on the triploblastic arrange- 

 ment of invertebrates, and raises it to the rank of a primitive organ under the 

 name of acroblast, and gives it an equal value with the other germinal layers. 

 He points out that acroblast exists in Aves and Lacerta as a peripheral 

 thickening between the epiblast and hypoblast before the mesoblast (the 

 archiblastic mesoblast of Waldeyer) is formed. The cells in this thickening 

 give rise to a series of amoeboid wandering cells (poreutse) by division, and 

 these in their turn fill in the serous cavities between the other germinal layers, 

 and form the blood and connective tissue. 



My own Observations. — I will now describe the changes which take place in 

 the parablast, as I have observed them in the herring and other forms. 



In the preceding section I described the appearance of the parablast at the 

 end of what I consider the 'primary segmentation stage in the herring. The 

 parablast, which has increased very considerably in bulk at the expense of the 

 yolk, leaves the periphery, and collects mainly under the archiblast. About 

 twenty-six hours after fertilisation transverse sections of the egg present the 

 appearance shown in fig. 12. The archiblast has become differentiated into two 

 layers. The outer small and somewhat flattened cells, which stain deeply with 

 carmine, constitute the epidermal layer of the epiblast. The cells more centrally 

 situated are larger, more rounded, and do not stain so deeply. They are loosely 

 aggregated together, and represent the nervous layer of the epiblast in other 

 Teleostean types which I have examined. It must, however, be remembered 

 that we have not yet arrived at the invagination stage, and the germinal layers 

 will not be differentiated for some time. Beneath the archiblast the parablast 

 appears as a thick layer of protoplasm which is undergoing division into cells. 

 Clear vacuole-like spaces are recognisable at irregular intervals both in the 

 peripheral parablast and in the part more centrally situated. Around these 

 clearer spots the protoplasm is becoming divided off so as to form a number of 

 cells. The lines of fainter colour in the parablast represent the planes of 

 division. So far as I can make out, there is no karyokinetic figure during 



