168 VERTEBRATE ZOOLOGY 



lively small egg of this fish, the first two cleavages are meridional and 

 divide the egg completely into four equal blastomeres. The third 

 cleavage is also meridional; but the fourth is equatorial, cutting off 

 eight micromeres and eight macromeres. The micromeres multiply 

 more rapidly than the macromeres and produce a blastula with num- 

 erous small cells at the animal pole and fewer large cells at the vegetal 

 pole (Fig. 97). The cavity contains many rather loose yolk cells. 

 Gastrulation occurs by invaginating the endoderm to one side of the 

 yolk mass. Embryo formation is very like that of the Amphibia. 



The extreme type of meroblastic cleavage is seen in the teleosts 

 (Fig. 98). Here the protoplasmic parts of the egg during the processes 

 of maturation migrate to the animal pole and round up into a yolk- 

 free hemisphere. This hemisphere undergoes cleavage, forming a 

 lens-shaped cap of cells. This so-called embryonic disk then pro- 

 ceeds to surround the yolk by a process of overgrowth or peripheral 

 spreading of the germinal disk, meanwhile differentiating an embry- 

 onic head. The germ-ring proceeds past the equator of the egg and 

 then grows together as it becomes narrower to form the embryonic 

 axis. Finally the ring closes completely, its substance having con- 

 cresced to form the embryonic body! This process of embryonic 

 concrescence is characteristic of all vertebrate embryos in which 

 meroblastic cleavage occurs. There are all connecting stages be- 

 tween the type of development seen in Neoceratodus and that seen 

 in the typical teleost. 



APPENDIX TO FISHES 



THE OSTRACODERMI 



Incidental mention has already been made to the Ostracodermi 

 as an early specialized group of pro-fishes. They are Palaeozoic 

 forms which have a wide range of characters, so wide in fact that it is 

 doubtful whether it is justifiable to place them in a single group. If 

 we give the group as a whole the value of a class coordinate with 

 Cyclostomata and Pisces, we may be justified in dividing this " class" 

 into a number of orders. 



ORDER I. HETEROSTRACI 



Of these forms we have little knowledge except of their external 

 features. They were evidently broad, flat creatures, more like our 



