EARLY DEVELOPMENT 41 



mandibles. The part lying in front of the antennae, however, has been 

 variously interpreted: as a primary head segment by Weismann (1864), 

 Heymons (1895a), and Heider (1889) ; as an ocellar segment by Viallanes 

 (1891) and Packard (1898); as an ocular segment by Holmgren (1908); 

 as the acron plus the preantennal segment by Heymons (1901); as the 

 brain and labral segments by Patten (1884), Wheeler (1889), and Carriere 

 (1898); as an acron, labral, and preantennal segment by Wiesmann 

 (1926); and as labral, clypeopharyngeal, and frontoocellar segments by 

 Verhoeff (1905). Furthermore, Folsom called the part in front of the 

 antennae the "oral segment" but recognized an interpolated superlingual 

 segment between the mandible and maxillae. Finally, Janet accounted 

 for four in front of the antennal segment, or nine in all. The earlier 

 investigators, whose works have been reviewed by both Wiesmann 

 (1926) and Eastham (1930), used the appendages, the mesoblastic somites, 

 and the neuromeres as the criteria indicative of segments. The attempts 

 at homologizing have been based on a study of both the embryo and later 

 stages; but since in certain cases some of the parts are purely embryonic 

 and in other cases much reduced or represented in varying degree in 

 different species of insects, it is small wonder that there are such diverse 

 opinions and that the number of segments attributed to the head of an 

 insect by the different workers should vary between six and nine. 



A departure from the former approach to the problem is found in the 

 comparative studies by Holmgren and Hanstrom on the annelid and 

 arthropod brain and in Solland's study on the crustacean genus Leander. 

 These workers strongly maintain that the postoral segment of the second 

 antennae is the first true somite, a view also shared by Snodgrass (1935, 

 1938). The principal ground for the generally accepted belief that the 

 acronal region of the arthropod embryo contains one or more ''cephahzed 

 somites" is the occurrence of temporary coelomic sacs in this region. To 

 this Snodgrass (1938) replies: 



It has not been shown that the presence of cavities in the cephalic mesoderm 

 is necessarily indicative of somites, and it would seem that the burden of proof 

 should be on the positive side of this question. . . . Coelomic cavities formed in 

 the cephalic region . . . being radial in position the cephalic sacs cannot repre- 

 sent "somites" in the manner of the paired sacs lying posterior to the mouth, 

 which are transversely opposed to each other. Hence, the assumption that these 

 anterior sacs represent "cephaUzed somites" is inconsistent with the anatomical 

 conditions that arise in the acronal region. Moreover, as may be seen in a study 

 of the annelids, the coelomic sacs themselves do not determine metamerism; the 

 segmentation of the postoral parts of the mesoderm bands is secondary to 

 metamerization of the primary somatic muscular system, and the coelomic 

 cavities are later formed. . . . The usual absence of well-differentiated coelomic 

 sacs in the annelid prostomium and the fact that the fullest development of the 



