The Embryology of Heterodontus japonicus 731 



figure 48a and 48b. In the latter figures the nucleus appears double, and in Text-figure 

 48c it is notched. These are indications of the duplex character of the nucleus, which 

 consists of both maternal and paternal components. Had the preceding stages of mitosis 

 been favorably oriented, doubtless the duplex organisation of the nucleus would have been 

 revealed there also. At the time when Dean's drawings were presumably made, obser- 

 vations of this kind on the segmenting eggs of vertebrates were rare. For a fairly adequate 

 bibliography of the literature on the individuality of the germ nuclei and the history of the 

 chromosomal vesicles during cleavage, the reader is referred to the contributions of Rich' 

 ards (1917) and Smith (1929). 



In an early section of this article, mention has been made of a few slides bearing 

 serial sections of embryos of Heterodontus (presumably japonicus). These include sagittal 

 sections of three blastoderms in early, advanced, and late blastula stages respectively. In 

 the sections, which were cut in paraffin, the early blastula measures about 1.1 mm. long, 

 the advanced blastula 1.3 mm., and the late blastula about 3 mm. In life these blastulae 

 must have been appreciably larger, since such material shrinks during the process of 

 preparing it for sectioning. In all essential respects the two earlier blastulae, as represent- 

 ed in sections, are like those of other Selachians: e.g., Torpedo and Pristiurus as portrayed 

 by Riickert (1899, Figs. 51 and 53, pi. LVI). They closely resemble the corresponding 

 stages of Heterodontus phillipi, discussed in the final paragraphs of this section. The late 

 blastula is imperfect, so that comparisons are unprofitable. 



In a brief contribution, Haswell (1916) describes surface views of two very early 

 stages of cleavage in the germinal discs of eggs taken from oviducts of H. phillipi 

 (my Text-figure 49a and 49b). He states that in both eggs the cleavage lines are entirely 

 confined to the area of the orange spot, and do not show any trace of a tendency to become 

 extended beyond its limits. Two other eggs, taken from "uteri" some weeks later, showed 

 more advanced stages of cleavage. Neither in these eggs, nor in those studied in 1898 

 (described in the next paragraph), did Haswell find any indications of furrows such as 

 those of H. japonicus interpreted by Dean as a reminiscence of holoblastic cleavage. 



Haswell (1898) studied the later stages of cleavage in H. phillipi from eggs that had 

 been deposited in the sea. He states that during cleavage the blastoderm appears as 

 a circular reddish-orange spot, around which is a narrow light-yellow band. When this 

 orange spot has attained a diameter of about 2 mm. it assumes an oval shape, its longer 

 axis corresponding with the future long axis of the body. At its posterior end appears 



B 



Text-figure 49. 

 Surface views of early cleavage in two blasto- 

 derms, A and B, of Heterodontus phillipi. The 

 eggs were taken from the oviducts. 

 After Haswell, 1916, Fig. 1. 



