NO. 14 INSECT ABDOMEN SNODGRASS 3 



To enumerate the segments consecutively would not be a difificult 

 matter were it not for the uncertainty, or difference of opinion at 

 least, concerning the number of somites that enter into the composi- 

 tion of the head. The discrepancy of opinion arises principally from 

 a difference of interpretation concerning " segmentation " in the 

 cephalic region anterior to the somite of the tritocerebral brain lobes. 

 This somite carries the second antennae, or chelicerae, and is without 

 question postoral in its origin, though its lateral parts may lap for- 

 ward at the sides of the mouth and its appendages thus acquire a 

 preoral position in the adult. The head region anterior to the trito- 

 cerebral somite, often called the acron in the embryo, has been sup- 

 posed to include an antennal. a preantennal, and even a labral somite, 

 the evidence adduced being the presence of corresponding coelomic 

 sacs in the mesoderm associated with ganglionic centers of the ecto- 

 derm innervating the sensory and appendicular organs of the acronal 

 region. The first antennae (antennules) are the procephalic appen- 

 dages most commonly present in the arthropods, and the antennae of 

 Onychophora would appear to be their homologues in this group of 

 related animals ; but the embryonic position of the antennae relative 

 to the mouth is variable, in some cases the appendages are preoral, in 

 others adoral, and in others again they are slightly postoral. 



Segmentation in the articulate animals is closely associated at least 

 with the formation of paired coelomic sacs in the mesoderm. In the 

 embryology of the annelids, as is well known, the coelomic mesoderm 

 usually takes its origin from a pair of teloblastomeres situated at first 

 behind the blastopore. From these cells are proliferated forward in 

 the ventrolateral parts of the body of the larva two bands of meso- 

 derm, in which there may be formed several pairs of primary coelomic 

 cavities. The secondary somites of the worm added during or after 

 metamorphosis are generated from a zone of growth situated between 

 the last larval somite and the small terminal region of the body con- 

 taining the anus (periproct, pygidium, telson). The mesoderm of the 

 postlarval somites is in most cases proliferated also from the primary 

 mesodermal teloblastomeres, though in some it is said to have its 

 source in the teloblastic ectoderm (Iwanoff, 1928). The forward 

 growth of the mesoderm bands in the Annelida is arrested at the 

 mouth of the larva, and the first pair of coelomic cavities lies just 

 behind the mouth ; as a consequence there remains anterior to the 

 mouth an unsegmented region of the trunk, known as the prostomium 

 (fig. I A, Prst). With later development, however, the interior of 

 the prostomium may become completely occupied by extensions of the 



