1904.] NATURAL SCIENCES OF PHILADELPHIA. 751 



of the formation of the larva; he noted the proboscis invagination, 

 and the spicules (''filaments") of the armature. 



Villot (1874) was the first to give an extended account of the develop- 

 ment of the larva. He described the formation of the egg-string 

 ("nid amentum"), and the appearance of the egg-envelopes; and 

 describes the polar bodies as being very variable in number, form and 

 volume. He found the segmentation to be total and equal and leads 

 to a "germe" formed of two concentric spheres (the process of which 

 was not determined), ectoderm and entoderm respectively. He noted 

 the ectodermal invagination at the anterior end of the embryo and in- 

 terpreted it to be the beginning of the "head" (equivalent to my term 

 "proboscis"). His fig. 49 is the most detailed sketch yet given of the 

 structure of the larva. The whole armature of the proboscis is de- 

 scribed very accurately. But he describes an anterior mouth com- 

 municating by an oesophagus with a posterior intestine, and the latter 

 by an open anus with the exterior — failing to see the diaphragm, and 

 mistaking for an oesophagus the solid axial cord of the proboscis. He 

 saw correctly the gland and its duct, but interpreted it as an excretory 

 organ. The musculature is noted, and the body-cavity described as 

 filled with cells. His second paper (1891) is mainly a criticism of the 

 work of Camerano. 



Camerano (1889) studied the development not quite to the point 

 of the larval stage, figures the chromosomes and certain details in the 

 formation of the polar bodies, which he finds to be two in number. 

 The earliest penetration of the spermatozoon he did not see, but 

 describes a succession of stages of the two pronuclei, and figiu-es them 

 as of unequal size. The early cleavage he finds to be quite variable in 

 different eggs, though holoblastic. In regard to the process of gastru- 

 lation his results are entirely different from mine. He finds the embryo 

 at the end of the cleavage to be a flattened plate, not a sphere, a disk 

 composed of two layers of cells— a sterroblastula. This plate becomes 

 quadrangular instead of circular in outline ; then an invagination takes 

 place at one end of the plate, which he interprets as a prostoma, and 

 concludes the sterroblastula to have become a coelogastrula, I can 

 only interpret Camerano's results as follows: First, that the strong 

 fixatives employed produced flattening of the embryos, which I have 

 frequently observed after fixation with acetic acid or strong alcohol; 

 second, that he did not see the gastrula invagination at all (which 

 would take place at the stage of his figs. 43 to 45), and mistook the 

 ectodermal proboscidial invagination for the gastrula cavity. 



Villot was in error in describing the true larva as the "etat embry- 



