THE MALAYAN SPKOIES OK ONYCHOPHOHA. 45 



in front it is being gradually encroaclied upon by the double 

 Liyer of cells wliicli lias grown across the middle portion of 

 the elongated opening found in the younger embryo. From 

 the middle point of tlie posterior border of tlie blastopore, the 

 primitive groove extends backwards for a considerable dis- 

 tance. The groove in question is not so evident in the 

 immediate neighbourhood of the blastopore as it is some 

 distance behind it. On each side of the groove and close to 

 the blastopore there is a triangular-shaped thickening, in 

 which very active proliferation is going on. These thickenings 

 constitute the so-called primitive streak, and are the sources 

 from which the mesoblastic bands are produced ; in fact, they 

 may be described as teloblastic spots from wliicli the 

 mesoderm is derived. 



The next stage of development to be considered is repre- 

 sented by the third figure on PI. 5. Unfortunately there 

 is a considerable gap between this stage and the previous 

 one. The posterior end of the embryo has grown round the 

 head, so that it almost touches the rudimentary antennae, which 

 at this stage consist of three rings. The rudiments of perhaps 

 all the appendages are visible ; those of the jaws and oral 

 papillee being specially well developed in comparison with the 

 others, which decrease in size from before backwards. The 

 brain lobes constitute a marked feature of the embryo at this 

 stage. Neither the body nor the rudimentary appendages 

 exhibit ring-shaped markings. 



The next stage to be described is illustrated by the fourth 

 figure on PI. 5. The posterior end of the embryo under 

 consideration has grown over the head, and the antenuje, 

 having increased in length, consist of about a dozen rings. 

 The brain lobes are enormously large in proportion to the 

 other parts of the body, and behind them are seen rows of 

 papillae, which represent the rudiments of the lips. The 

 oral papilla3 present at their free ends a marked depression, 

 vrhich represents the opening of the future slime-gland. 

 Both the body and the appendages are pi-ovided with ring- 

 like markino-s. 



