564 THE STRUCTURE AND DEVELOPMENT 



stances two central nucleoli are present, in which case the regu- 

 larity is considerably interfered with. The points in which the 

 youngest permanent ova differ from the primitive may be 

 summed up as follows : 



(i) The permanent ova are larger, the smallest of them 

 being larger than the average primitive ova in the proportion of 

 four to three. (2) They have less protoplasm as compared to 

 the size of the nucleus. (3) Their protoplasm is granular instead 

 of being clear. (4) Their nucleus is clear with exception of a 

 network of fibres instead of being granular as in the primitive 

 ova. It thus appears that the primitive ova and permanent ova 

 are very different in constitution, though genetically related in 

 a way to be directly narrated. 



The formation of permanent ova is at its height in embryos 

 of about seven centimetres or slightly larger. The nests at this 

 stage are for the most part of a very considerable size and 

 contain a large number of nuclei, which have probably, as before 

 insisted, originated from a division of the smaller number of 

 nuclei present in the nests at an earlier stage. Figs. 14 18 are 

 representations of nests at this period. The diameter of the 

 nuclei is, on the whole, slightly greater than at an earlier stage. 

 A series of measurements gave the following results : 



o - oi6 mm. 

 O'oi6 mm. 

 0*018 mm. 

 O'O2 mm. 

 o'O2 mm. 



Both varieties of modified nuclei are common enough, though 

 the stellate variety predominates. The nuclei are sometimes in 

 very close contact, and sometimes separated by protoplasm, 

 which in many instances is very slightly granular. In a large 

 number of the nests nothing further is apparent than what 

 has just been described, but in a very considerable number one 

 or more nuclei are present, which exhibit a transitional character 

 between the ordinary stellate nuclei of my second category, and 

 the nuclei of permanent ova as above described ; and in these 

 nests the formation of permanent ova is taking place. Perma- 

 nent ova in the act of development are indicated in my figures 

 by the letters d o. Many of the intermediate nuclei are more 



