518 EMBRYOLOGY OF THE LOWER VERTEBRATES oh. 



germinal disc comes to be represented by a lenticular blastoderm 

 lying at the apical pole of the egg and corresponding to the mass of 

 micromeres of such a holoblastic egg as that of Lepidosiren. The 

 superficial layer of cells become fitted closely together and form a 

 definite epithelium — which is destined to become the ectoderm. The 

 cells of the lower layers on the other hand are rounded with chinks 

 between them representing the segmentation cavity. The lowest of 

 all have the appearance of being incompletely cut off from what is 

 ordinarily termed the white yolk lying below them but which is really 

 a syncytial layer full of fine granules of yolk and with scattered 

 nuclei. 



Apparently a few accessory sperm nuclei are usually present in 

 the fertilized eggs and faint traces of abortive segmentation may be 

 visible round them (cf. Elasrnobranch, Fig. 8, B*, p. 14). 



At the time of laying the blastoderm forms a small whitish disc 

 covering the apical pole of the egg. Sections show it to consist of an 

 upper layer of ectoderm and of a lower layer consisting of numerous 

 rounded micromeres lying about in the fluid of the segmentation 

 cavity. These micromeres become larger towards the lower face of 

 the blastoderm and they are more crowded together round the 

 periphery. 



It must not be supposed that all newly-laid eggs show exactly 

 the same degree of development. As a matter of fact great variation 

 occurs, one of the chief variable factors probably being the length of 

 time occupied in the passage down the oviduct. Where this time is 

 longer, as e.g. towards the end of the laying season, the stage of 

 development of the egg when laid is more advanced. 



The First Day of Incubation. — After the egg has been laid the 

 lowering of the temperature leads to such a slowing of its vital 

 processes that development appears to come to a standstill. If kept 

 at a low temperature it retains its vitality for a considerable period 

 but makes no appreciable advance in development. If the tempera- 

 ture be raised by incubation the developmental processes are at once 

 accelerated and comparatively rapid changes come about. The 

 blastoderm increases in size, its margin spreading outwards, and at the 

 same time there comes about a distinct difference in appearance 

 between its central and marginal parts — the central portion assuming 

 a dark transparent appearance (pellucid area) which contrasts 

 strongly with the whiter " opaque area " surrounding it. The 

 examination of sections at once explains this difference in appearance : 

 the more opaque appearance peripherally is seen to be due to the 

 lower layer cells being there closely crowded together. 



An important change soon comes over the lower layer cells, 

 in as much as those next to the yolk, in the region underlying 

 the pellucid area, lose their rounded shape, become somewhat 

 flattened and adhere together edge to edge to form a continuous 

 membrane — the (secondary) endoderm. This appears first beneath 

 the posterior portion of the pellucid area ; it gradually extends 



