THE SEGMENTATION OF THE OVUM. 119 



From observations of my own I am inclined to differ from Ludwig as to 

 the nature of the parts within the blastoderm. My observations have been 

 made on Agelena labyrinthica and commence at the close of the segmenta- 

 tion. At this time I find a superficial layer of flattened cells, and within 

 these a number of large polyhedral yolk cells. In many, and I believe all, 

 of the yolk cells there is a nucleus surrounded by protoplasm. It is generally 

 placed at one side and not in the centre of a yolk cell, and the nuclei are so 

 often double that I have no doubt they are rapidly undergoing division. It 

 appears to me probable that, at the time when the superficial layer of proto- 

 plasm is segmented off from the yolk below, the nuclei undergo division, and 

 that a nucleus with surrounding protoplasm is left with each yolk column. 

 For further details vide Chapter on Arachnida. 



Although by the close of the segmentation the protoplasm 

 has travelled to a superficial position, it may be noted that at 

 first it forms a small mass in the centre of the egg, and only 

 eventually assumes its peripheral situation. It is moreover clear 

 that in the Spider's ovum there is, so to speak, an attempt at a 

 complete segmentation, which however only results in an 

 arrangement of the constituents of the ovum in masses round 

 each nucleus, and not in a true division of the ovum into distinct 

 segments. 



It seems very probable that Ludwig's observations on the segmentation 

 of Spiders only hold good for species with comparatively small ova. 



In connection with the segmentation of the Insects' ovum and allied 

 types it should be mentioned that Bobretzky, to whose observations we are 

 largely indebted for our knowledge of this subject, holds somewhat different 

 views from those adopted in the text. He regards the nuclei surrounded by 

 protoplasm, which are produced by the division of the primitive segmenta- 

 tion nucleus, as so many distinct cells. These cells are supposed to move 

 about freely in the yolk, which acts as a kind of intercellular medium. This 

 view does not commend itself to me. It is opposed to my own observations 

 on similar nuclei in the Spiders. It does not fit in with our knowledge of the 

 nature of the ovum, and it cannot be reconciled with the segmentation 

 of such types as Spiders or even Eupagurus, with which the segmentation in 

 Insects is undoubtedly closely related. 



The majority if not all the cases in which a central yolk mass is formed 

 occur in the Arthropoda, in which group centrolecithal ova are undoubtedly 

 in a majority. In Alcyonium palmatum the segmentation appears however 

 to resemble that of many insects. 



One or two peculiar varieties in the segmentation of ova of this type 

 may be spoken of here. The first one I shall mention is detailed in the 

 important paper of E. Van Beneden and Bessels which I have already so 

 often had occasion to quote : it is characteristic of the eggs of most of the 



