Please answer a question about the following article about Macintosh:

Smith's first Macintosh board was built to Raskin's design specifications: it had 64 kilobytes (kB) of RAM, used the Motorola 6809E microprocessor, and was capable of supporting a 256×256-pixel black-and-white bitmap display. Bud Tribble, a member of the Mac team, was interested in running the Apple Lisa's graphical programs on the Macintosh, and asked Smith whether he could incorporate the Lisa's Motorola 68000 microprocessor into the Mac while still keeping the production cost down. By December 1980, Smith had succeeded in designing a board that not only used the 68000, but increased its speed from 5 MHz to 8 MHz; this board also had the capacity to support a 384×256-pixel display. Smith's design used fewer RAM chips than the Lisa, which made production of the board significantly more cost-efficient. The final Mac design was self-contained and had the complete QuickDraw picture language and interpreter in 64 kB of ROM – far more than most other computers; it had 128 kB of RAM, in the form of sixteen 64 kilobit (kb) RAM chips soldered to the logicboard. Though there were no memory slots, its RAM was expandable to 512 kB by means of soldering sixteen IC sockets to accept 256 kb RAM chips in place of the factory-installed chips. The final product's screen was a 9-inch, 512x342 pixel monochrome display, exceeding the size of the planned screen.

What feature was missing from the final Mac design produced by Smith?
memory slots