“ PROBOSCIS PORES ” ]N CRANIATE VERTEBRATES. 541 
alternating with the large ciliated cells, and also round the 
opening of the pit. Here the rods become shorter and 
shorter, until this type of cell passes into the more ordinary- 
ciliated epithelium of the organ of Muller (PI. 28, fig. 12). 
Hatschek, who noticed the rod-bearing cells, considered 
the pit to be a sense-organ. But since no nerve can be 
traced to it, this interpretation is probably incorrect. Andrews 
states that in Asymmetron the pit secretes a mucous sub- 
stance, which entangles food-particles and gets carried into 
the mouth. He points out its relation to the blood- vascular 
■“glomus,” and concludes that Hatschek’s pit is a slime- 
secreting gland — a conclusion later supported by van Wijlie. 
The story of the origin of Hatschek’s pit is one of the 
strangest episodes in the strange history of the development 
of Amphioxus. It is a mesoblastic structure formed from the 
first mesoblastic somite of the left side. Hatschek (5) studied 
the development of the anterior pair of pouches, and correctly- 
described that on the right side as enlarging forwards and 
downwards so as to give rise to the main head-cavity of the 
larva. The left pouch he believed became constricted into 
two portions. One, taking up a position on the right and 
below the notochord, gave rise to Hatschek’s pit itself, while 
the other opened to the exterior on the left side and gave 
rise to the preoral pit of the larva (6). The larval preoral pit 
subsequently becomes drawn into the buccal cavity at meta- 
morphosis, and acquires its definitive position in the adult by 
a process of shifting and overgrowth admirably depicted by 
Willey (15). Hatschek's description of the early development 
of the pit is by no means clear, and unfortunately is pub- 
lished without figures (6). Some years later, Legros (9) 
stated that the sac on the left side of the early embryo was 
derived, not from a coelomic pouch, but from an invagination 
of the ectoderm, and that from it were developed the pit, the 
ciliated organ, and the anterior nephridium (Hatschek’s 
nephridium). 1 This interpretation of the origin of the sac 
1 This nephridium is developed neither from a mesoblastic funnel, 
as described by Hatschek (6), nor as an outgrowth from the second 
