72 CLAVPOLE. [Vol. II. 



to form a "dorsal organ " similar to the structure of that name 

 in the pterygote insect. The author proves two points: (i) 

 that Thysanuran cleavage is superficial, differing from the Col- 

 lembolan type, and (2) that embryonic membranes are formed 

 homologous with those of the Pterygota. Hence Lepisma is 

 an intermediate form transitional between the Collembola and 

 higher insects. 



This paper is followed in 1897 by a longer and more com- 

 plete study of the same form at the hands of the same worker, 

 Heymons.^ In this he shows that some of the cleavage nuclei 

 migrate from the center to form the blastoderm, while others 

 remain in the yolk as yolk-cells. The gastrula has the form of 

 a circular depression instead of the typical groove, and as soon 

 as a two-layered condition is attained the germ band sinks into 

 the yolk. While buried in the yolk the germ-band segments 

 and paired appendages appear. First antennae, post-oral in 

 position, next distinct intercalary appendages, mandibles, and 

 two pairs of maxillae with a median unpaired labrum. The 

 maxillae early split in two longitudinally, and the maxillary 

 palps remain clearly homologous with the thoracic legs. Paired 

 abdominal appendages appear on each segment except the i ith, 

 getting progressively smaller from the first pair to the loth. 

 After several weeks the larvae hatch and are chiefly distin- 

 guished from the adults by their white color and the absence of 

 the styli and cerci. 



The reproductive cells appear at an early stage in the hind 

 end of the embryo and are clearly of ectodermic origin ; after 

 much migrating they enter the primitive somites and form 

 follicles segmentally arranged in the female. The mesenteron 

 was described as arising from yolk-cells that migrate from the 

 yolk and multiply to form a continuous layer enclosing the 

 yolk and is hence entodermal in origin. In conclusion it is 

 clear that the Thysanura show strongly marked pterygote 

 peculiarities, and the conditions described suggest the author's 

 opinion that the formation of embryonic envelopes is due to 

 increase of yolk material in the egg. 



^ Heymons, R., " Entwicklungsgeschichtliche Untersuchungen an Lepisma sac- 

 charina L.," Zeitschr.f. wiss. Zool. Bd. Ixii. 1S97. 



